I have a problem. When ever I march left or right I tend to turn my body in the direction i'm marching slightly and I know I need to keep my upper body straight to make our line look clean.

Has anyone had this problem and if so, do you know how to fix it?

oh, I almost forgot, I march snare.

you can fix marching technique the same way you fix playing technique. Do it slowly, look in a mirror if you have to. With your drums on, figure out what you need to do to keep your hips straight while crabbing. The hips being straight is key. In case you haven't been taught this, when you crab the foot that's crossing over is placed sightly in front of the other foot, so that the two feet are sliding sideways in different tracks. Also make sure you stay up off of your heels, and that your feet stay pointed straight. Like I said, just do it slowly by yourself and learn what movements are used to keep your hips straight. It will take practice just like playing does.

Like he said, do it slow.

Another thing you can do is to check out the front of your drum instead of focusing on your body while you're moving. Pointing your instrument in the right direction is what you're after.

I will try these things, I wouldn't want to make matt (the center snare this year) angry again.....

Most people make this mistake because the feet turn slightly in the direction that they are moving naturally. You need to make a conscious decision to keep your feet straight and your body upright. I've seen that problem a few times in our line, and that seems to correct it.