I've been on a single plane for as long as 25 hours (Atlanta -> Johannesburg, 4 hour delay before takeoff, and stop for fuel in Senegal without the ability to get off the plane, and while I'm in coach). The seats aren't that bad if you pick a good one.
Here's where you do seat research
http://www.seatguru.com/
I was just about to post the same thing. That website (and a few other websites like it) show which seats are good, marginal, or bad.
Then I do things like this:
1) I get window seat so that I have something to lean on. Leaning helps for comfort quite a lot. I don't care if I wake others or accidently elbow them in the face when I need to go to the bathroom. They should know better and get a window the next time.
2) If the window isn't available, then get the aisle. A good trick is that almost all isle armrests are movable (check for a hinge). There is a hidden secret button underneath the armrest (often in a hole on the bottom side of the armrest). Press the button, pull the armrest up about 30° to 60° and bingo, you can now sleep or rest leaning into the aisle against the armrest (a pillow or coat or blanket helps to distribute the weight).
3) Oscillate from one side of the plane to the other if you have multiple flight legs. That way, you get to lean on a different side with each flight leg.
4) Keep your seat back angled differently from those around you. It either puts you ahead of them for armrest control or it puts you behind them so you can lean on their seat side.
5) Select an undesireable row. You'll be far more likely to have an empty seat next to you.
6) Select the 2-seat section not the 3-seat section, if possible. Couples want to sit together and the 3-seat sections are more likely to be full. Plus, you are guaranteed not to get that horrible "middle" seat that has no window to lean on and no aisle for quick escape.
7) Take your shoes off.
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