What type of under the seat bag is practical, and what should be stored in it for the flight?

The following is a list of the sort of items you should include in your cabin luggage (eg. not your checked luggage):

• All valuables (jewelry, money, etc)
• Important documents (passport, itinerary, hotel reservations, medical prescriptions, etc)
• All electronics (phone, laptop, mp3 players, etc)
• Anything fragile or which might break due to rough or careless handling
• Medication, especially any that you may need to take during the flight such as lactose pills, air sickness pills, pain relief for headaches, etc.
• Anything you may want to use during the flight

What goes under the seat should be the stuff you intend to use during the flight. This may include: some electronics, like a laptop or iPod; a travel pillow; books or other reading material; a blanket or jacket; essential toiletries (eg feminine hygiene products), and so on.

It's a matter of preference, of course, what you do on the flight. But if you think you'll want to listen to your iPod, it should go under the seat rather than in the overhead so you're not needing to go rummaging. (Going into the overhead on an overnight flight is especially difficult since the crew tends to dim or turn off the cabin lights, so you'll be doing said rummaging in the dark.)

What type of bag you use depends on what needs to go into it. If you're just taking your passport and an iPod and planning to nap most of the trip, you can get away with using a purse. If you're taking more, a backpack may be more practical. The type of bag also depends on what you're actually planning on using in your destination, since taking excessive luggage just for the plane trip isn't always practical.

But in general, the bag should be soft so that it can be fit under the seat and potentially squished a bit. It shouldn't be excessively large in any dimension -- check with your airline for dimensions. And it should be relatively easy to get stuff in or out of it. (Eg a zipper rather than a system of six buckles.)

A few bonus tips to go with Roddy of the Frozen Peas excellent answer:

• Don't make the bag too big, as it will be taking up some of your legroom. Some aircraft have electronics boxes for the in-flight entertainment system under the seat, which further reduces the available space. You can always put it in the overheard if there's space, particularly if you have an aisle seat. With a window or middle seat, I'll usually pull the items I need out of my bag before sitting down and put the bag in the overhead, space permitting, but that's up to you.

• Make sure to bring any chargers for your electronics. There may be outlets available at your seat.

• Try to designate a particular place in your bag for your travel documents (passport, boarding pass, etc...) so you can find them easily when you need to. Going along with that, don't forget your passport is in your bag if you've zipped it into a special compartment.

• For long flights, especially overnight ones, consider bringing some basic toiletries to freshen up before arrival. This may involve a comb, razor, toothbrush, etc...

• Remember that your checked luggage could be delayed or even lost, or you may experience flight delays that leave you without access to your bags for an extended time. Carry with you anything that you must have upon your arrival or at your destination, even if you won't need it on board the plane. This is especially true if your itinerary might involve a close connection or multiple changes of planes, as there's a higher chance of being separated from your bags under such circumstances.

• If I'm checking the majority of my luggage and just bringing a small personal bag on board, I usually like to carry at least some emergency clothes in my carry-on in case my luggage gets lost. Being separated from your luggage is never fun, but it's a little better if you have a clean shirt and a pair of underwear ready for that eventuality. If you're giving an important business presentation and absolutely must wear a suit, don't pack your only suit in your checked luggage. (To those Italians at that New Year's gala where I showed up in baggy jeans and a not particularly clean t-shirt and you all assumed I'm an ignorant American who can't dress himself: I am, once again, sorry.)

• I like to bring an extra tote bag that folds up really tight (like this, only hopefully for far less than $64). Takes up minimal space going, but gives you room for purchases and extra stuff on the way home.

• Many countries (particularly the US and much of Europe) have restrictions on the quantities of liquids allowed in carry-on (not checked) baggage. If you're traveling through a location with such restrictions, don't carry more than the allotted limit. There should be an exception for medically necessary liquids.

• Even if your flight includes meal service, you may consider bringing some snacks along, especially if you often get hungry or are picky about food. For international flights, be sure to either dispose of any leftover food before going through immigration and customs, or declare it to the proper authorities upon arrival. (Australia is particularly strict about importing food. If you declare all food and show it to the inspectors, there shouldn't any any problem and they'll let you know what is allowed in, but you can be fined for trying to bring in food without declaring it).

When I'm on flights that long what I like to do is carry a larger backpack with a smaller bag inside of it that has the stuff I'll want in-flight. The larger backpack additionally has stuff that I want access to in an airport or in case of getting stranded overnight from a flight cancellation or missed connection. Putting the smaller bag inside the larger one makes it easy to carry around the airport, then I take out the small one once on board so that I'm not using all of my legroom for the larger bag.

For the smaller bag that goes under the seat in front of me, I prefer to use a cloth bag or one which otherwise changes shape very easily so that it takes the shape of whatever's inside it and, thus, doesn't waste more space than necessary (and can also be shoved into spaces of various shapes.) It doesn't need to be a particularly sturdy bag, since it will be inside the other larger (and sturdier) bag when walking around the airport, going through security, etc. Women who typically carry a purse may wish to just carry that, but I'm not a woman and don't carry a purse, so I can't speak to how well that works.

What I prefer to put in the smaller bag is very similar to what others have suggested:

• Tablet
• Chargers for phone and tablet
• Headphones - These can be used for your own devices and also usually for the airplane's IFE system. These can also be very helpful for sleeping in case of crying children nearby.
• A very small travel power strip (since aircraft that have AC power usually just have one plug available, sometimes even shared between adjacent passengers.)
• Passport
• Pen (for filling out immigration arrival card, which the flight attendants will usually hand out before you land)
• Small, light snacks (in case I want something in the middle of the night and/or I don't deem the airplane food to be of edible quality)
• Water bottle (which I fill in the terminal before boarding)

Note: Most countries allow you to take an empty water bottle through security and fill it inside the terminal after passing security (or to buy a filled one after passing security) to take on-board. Certainly, this is the case for flights originating in the U.S. However, it might not be the case for flights from Australia to the U.S.

Other items you might want in it:

• Phone (I usually have mine on a belt clip instead)
• Wallet (I usually have mine in a pocket instead)
• Medicine that you'll want/need in-flight

All of the above fits inside a relatively small bag that I can put under the seat in front of me without impeding my legroom much (which, at 6'1", is extremely important for me on a long-haul flight.)

Once the aircraft has reached cruising altitude, I sometimes move the bag back under my legs if I'm sitting in a window seat where it won't be in anyone else's way. This leaves all of my leg space open (unless I'm unfortunate enough to get stuck in a seat where legroom is blocked by an electronics box for the IFE system - a situation which I usually avoid by checking seatguru ahead of time and choosing my seat accordingly.)

As previously mentioned, the larger bag has stuff I might want access to in airports and/or if I get stranded overnight due to problems with the flights. For long-hauls, this typically includes things like:

• Change of clothes
• Travel-sized shampoo, toothbrush and toothpaste, comb, etc.
• More snacks (to eat in the airport in lieu of absurdly-overpriced airport food or to restock the in-flight bag between flights.)

These can take up a lot of room (relative to the size of the space under the seat in front of you, at least) and aren't typically needed in-flight, so there's no reason for them to be wasting your legroom.

Anything valuable and all important documents for your travel should be as close to you as possible. This means your passport, boarding passes (unless using electronic ones but I will bring a printed copy too), address at your destination plus any other necessary documentation to pass immigration and customs at your destination airport, including potential layovers and terminal changes which can force you to go through immigration in a country which is neither your departure nor final destination.

Other answers cover some details plus also add things you want during the flight such as snacks and entertainment, space permitting. Make sure you have any prescription medications with you, and going to some destinations, you may need a copy of a prescription or ordnance for the medication. Should there be some delay, this and any additional carry-on, may be your only items for a while. Since it can happen to have unscheduled over-night stops, due to bad weather or missed connections or airplane mechanical issues, I also recommend a change of clothes, at least socks and underwear, plus a few toiletries (toothbrush, toothpaste, deodorant and wipes at least).

As for the bag, Most airlines allow a carry one and a personal item. The personal item has a smaller size limit, so that is the one that I place under the seat. The carry-on goes in the overhead bin where it will be slightly less accessible but arrives most of the time intact at your destination. It happens though that due to limited space, the carry on gets checked at the gate or even to a faraway bin. That's why I always keep the most valuable and essential under the seat in a personal item sized-bag.

Most time I use quick access shoulder bag but lightweight backpack can do. A number of these have a dedicated padded compartment for a laptop, ultra-book or tablet which count as your valuables if you travel with such thing.

Under the seat? Hopefully nothing. That's where I store my feet. There's not enough space for a bag.

To add to all the great answers given;

• It can be very cold in the airplane, I like to take a pair of warm socks with me.

• The air can be very dry and rough on the skin, I prefer to take a moisturising cream and lipbalm with me, so I don't start my holiday with itchy, cracked skin and lips.