Stand up?

Who stands up going up hills?

Roadies do....

Women, of course, do

Seriously,

I find that I go faster when I don't stand up.
(I might stand up, albeit it infrequently, at the very top of a particulary hard climb, when leveling off, to give my spin a 'different angle' and let my legs recover a bit ---the fixie equivalent to changing gears.)
The key is to relax and maintain good form in terms of spin.
Once you start pressing, your form falls apart, and it's all over.