How can I accurately estimate the weight of my luggage?

Weigh yourself on a home scale, then weigh yourself holding the bag. Subtract the first from the second and you'll get the weight of the bag. If the bag is bigger than the scale, such as hanging over the sides, it won't weigh properly and that's why you have to stand on the scales holding it.

As Simon suggested, you do not have to estimate and can buy a handheld scale which you place in your luggage after measuring if you intend to acquire things during you travel.

Prior to having one of those I estimated and was rarely off by more than a few pounds. The idea is very simple: learn what 50 lbs feels like. You have to do this at home by lifting your suitcase with various content and comparing to something known to weight 50 lbs. The easiest to measure against a 50 lbs dumbbell or my 5 year old who weighed 49 lbs :)

I've gone to a courier or post office in the local area in the past. They have large industrial-sized scales that will easily take your backpack/bag/suitcase/camel and give you an accurate reading of the weight. Sometimes it's even in the public area so you don't need to check with anyone,but otherwise ask and act like the crazy tourist, panicking over their suitcase weight - most people will be keen to help out and see if you're going to be over the weight - besides, if you are, you might be shipping stuff home and look - you're in a post office! :)

It does depend on where you are staying, of course, but I have found in the past that most mid-range-to-luxury hotels (and sometimes budget ones, too) will have a scale available, even if there isn't one in the room. Just ask at the front desk / concierge / etc.

If you don't have access to a scale and there's no post office around you and you don't know what 50 lbs feel like, just go to the airport one hour earlier, find an empty check-in counter and weigh it there. Plan enough time so you can repack some stuff to your hand luggage or even take out some of those heavy boots/jackets/etc. and wear it. Since all scales at the airport are calibrated, 50.0 lbs on one scale are also 50.0 lbs on another scale and you can make the most of your baggage allowance!

This is not the ultimate answer, but just an extra tip in case you don't have a scale and have no idea what to do:

I have seen my self in that situation more than once. I have an handheld scale, but it's not hard to forget it.

What I often do is to weight the luggage at the airport. I just go to an empty check in desk and put my luggage on the belt. Most times the scales are turned on and have a weight LCD screen for the user. I sometimes even do it if I weighted the luggage at home but I am near the limit. Home scales can have up to half kilo deviations and can be hard to read with precision if they don't have a digital screen.

I know weighting in the airport can be sometimes too late but at least you don't go through the embarrassment and pressure of having extra weight and all queue of people waiting for checking-in while you try to solve the problem (either by paying extra or removing stuff).

In any situation, if you don’t want to pay extra, prepare to leave things behind or to be able to quickly move things between hand luggage and checked-in luggage (sometimes only one of them is extra weight) or to some friend luggage that has less weight. Choose the items you wish to exchange or that you are prepared to leave behind and leave them in a position where they are accessible.

What I usually do is:

1. If it's a returning trip and I weighted my suitcase at departure (home or airport) I know how much weight I have available. I just try to put the same content (or less) and estimate the extra weight I want to put in (souvenirs, documents, whatever)
2. If I have no clue than it's bad, but I try to pick the heaviest things and put it on the hand luggage. Hand luggage can be weighted but there are several situations that make a bit easier to handle. A. some companies don't have weight restrictions for hand luggage; B. you can still do your check-in and than solve the extra weight problem after leaving the queue. You can than return to ask for the "approved hand luggage" sticker if that applies.

In any situation try not to arrive late to the airport so you have time to handle the situation.

To facilitate weighing of my travel luggage, I have made a simple aid. It has a calibrated portable spring scale, which hooks into a suitcase handle. The suitcase is lifted with the help of a two-meter lever until it swings clear and its weight is registered. The lever is supported by a chain and hook supported from a steel tubing arch. The arch fits over two vertical lengths of tubing that are welded to a bracket that is fitted with two legs. Due to the leverage, less power is required to lift heavier luggage or cases.