I try to practice in stints where I will play my best however I tend to play longer than that sometimes and my performance degrades the more I play, which is basically what Platinum is saying.
I guess just don't overwork yourself expecting to have incredible play the whole way through, we ARE human beings and in that respect we eventually tire out mentally and physically from any activity we do. So try not to go beyond that point.
And NEVER, NEVER keep playing when you're just building up lots of frustration by doing so or are already in a bad mood to begin with. It'll take the fun out of whatever you're doing, so try to be in a good mood when you practice and especially in matches where it counts.
I always find keeping the game "fresh" and "new" is something that helps me out a lot too, taking a couple days off (sometimes longer or shorter) and then coming back to TF2 I always see something different that I hadn't noticed before in my play.
Just my advice.