How can I reliably make myself a cup of coffee in my hotel room that doesn't suck?

There are combination French press/coffee mug thingies which are very practical. You only need coarsely ground coffee and *Just Add (Boiling) Water (tm). If you shudder at the thought of pre-ground coffee, you can complement this with a small manual coffee grinder.

Bonus points: you can use the same device to make tea as well.

Have you considered an Aeropress? I've been using one for around two years now, and I'm very happy with the quality of the coffee it produces.

As far as travel suitability goes, I think it would be a good fit for hotel room brewing, provided you have access to hot water and coffee grounds. It's compact enough to fit in luggage, easy to use and clean, and only costs around $30.

Cory Doctorow wrote a blog post about using the Aeropress as a cold-brew filter which might give you some ideas.

Nothing as subjective as what a decent cup of coffee constitutes. Some shiver at the thought of instant coffee, others (like me) shiver at the thought of cold brewed coffee.

If you like instant coffee life is easy as most hotels (at least in Europe) come with complimentary instant coffee and teabags.

For the grain coffee lovers, getting your daily shot becomes a bit more complicated, but only slightly. Often you'll find a coffee machine in the breakfast area. Outside breakfast hours, you'll probably be allowed to help yourself to a coffee if you ask at the reception. You might even just try pushing the button.
Often these machines produce good coffee.

If this doesn't help you getting a free coffee you can always pack your own bags of instant coffee. Or bring along filter paper and a coffee funnel.

EDIT There are also these cups you see so often at flea markets. I have no clue what they are called, but image search on google revealed that some call them "Vietnamese coffee filters":

. Given the abundance of these coffee filters on sale on French and Belgian fleamarkets, I doubt the Vietnamese origin though. I suspect some colonial French influence.

At every flea market I go to in either France and Belgium, I see them on sale. Anyway if you are interested in using one on your trips you might try to buy one on the internet. Try the term "koffiefilter" on for example http://www.2dehands.be or ebay.be

I would head to your local outdoor / camping store and see what they have. "Cowboy" coffee is no longer good enough for camping (not that it ever was).

I have a one-cup Lexan French press that makes very good coffee, and has the benefit of giving you a place to store a zip-loc bag of fresh grounds. I couldn't find an exact match, but here are some examples from a store that I frequent.

One thing that I would not recommend: you can buy a device that looks like a tea ball with finer mesh (again, couldn't find a picture). This is (barely) acceptable when backpacking. If you have the room for a press, it's not.

You can buy an electric moka which only requires some current and fresh water:

The coffee from mokas is stronger than coffee people usually drink in USA (or other countries outside EU/Latin America), since it passes through the ground coffee at higher pressure (about 1.5 atmospheres I believe). You can read more information on the wikipedia page.

They come with different sizes, the smallest one would be about 15 cm tall with a base of about 10x12 cm (at least, from the electric mokas I've seen/used). Depending on the space you have available it might be too big or small enough to pack.

The price depends a lot on the dimensions, the brand etc. but I have seen some at about €30 to €40 on Amazon/eBay and other websites.

Tried to figure this out for years.

My polish girlfriend then told me a trick how they made turkish coffee in Poland when they had nothing else than hot water, coffee and one (two) cups.

Put the coffee powder in one cup and fill it with boiling water as usual. Wait until the desired intensity of coffee taste has developed. Then tap or knock the cup on the table for two or three times. This will prompt the coffee powder to sink to the bottom of the cup and you have a wonderful coffee.

You can also fill the the coffee in a second cup when the powder is sagged to keep the taste.

If you have to use the coffee maker in the room, here's what I do:

1) Clean it out by running it with water only. Throw that water away.

2) The coffee makers don't tend to heat the water enough, so run it again with water and ...

3) Put the water that's been heated once into the reservoir, and then put the coffee grounds in - the water will be hotter and it will do a better job of extracting what little flavor there was.