should i stand up in corners?

nope

i dont think your spose to, the mx'ers dont do it they sit real far up on the seat

In corners you always sit down as far up to the front as you can. Im almost touching my gas cap with my crown jewells. Put your inside foot straight out toward the front of the bike,with a slight bend at your knee,to where it is touching the radiator shrouds. I usually have my leg up to about the middle of the radiator shroud, This is kinda weird because you want to stick it out to the side just above the ground but dont do that. Dont let it touch the ground or you could end up closer to the ground than you want to be :D Also put weight on the outside peg, that will make it track alot better and it feels more stable.

Standing is good for flat high speed corners and rutted, whopped sandy corners where you need a ton of leverage on the bike. Your body can exert MUCH MORE of a leverage factor over the bike while standing. Sit in all other corners. Ideally, you sit in the "pocket" of the seat, not all the way up and keep all of your weight on the outside peg and the seat. NO WEIGHT ON THE BARS!!! Remember to keep the ball of your foot on the peg too, that is really important.
Moo, sitting far up makes the rear end really fishsy. The pocket of the seat is a much better choice. You bike does not need much weight on the front end to turn well at all. I take all sand corners almost on the fender with my foeght on the pegs standing. Rutted corners I do sitting down with one foot out, etc.