I always seem to do very well in the beginning of tournaments, often doubling or tripling my starting stack in the first hour....After around two hours I start to get tired and lose focus.
Simple: If you want to be a decent tournament player then the most important thing you need is PATIENCE PATIENCE PATIENCE. If you do not have this trait as part of your poker playing arsenal then you're better off heading for the 6-player cash games or turbo SnGs.
If you often start well by doubling and/or tripling up then you're half way there. There is absolutely nothing wrong with throwing away 20, 30, 40, 50
hands in the middle stages as long as you're playing correct poker and your stack is healthy in proportion to the blinds. (The odd steal or limp might be required at times.)
Nevertheless I agree with other posts about music (I'll second chiefer's Pink Floyd recommendation). Music is a great way to keep you occupied through the lean spells and can often be highly motivational as well. TV is a no no - you have to keep your eyes on the action that matters.
I have to disagree with nubstyle about multi-tabling in the late stages of a tournament. That is definitely NOT the time to fire up more action. Why would you want to divide up your attention when you're so close??!!