Stand Strong
Before you ever start to spin your flag, make sure that you "look the part". This means that you are standing in second position (feet shoulder width apart), unless told otherwise, with your head up, relaxed shoulders, and a straight back. Also, hold your flag with a strong grip. In my guard, we were always told that if someone were to come by and push on your flag, you should be holding it so tightly that it wouldn't move. Acting like you are confident (whether or not you really are) will make a bigger impact on your audience and score major points with your judges.
Solid Stops
Solid stops - Whenever you stop your flag, whether on a catch or whatever, you should always stop it with a very strong grip. What this means is that you should stop it so that it doesn't wobble back in forth in your hands. If it is a two-handed stop (or catch) you MUST catch it with both hands at the same time... if your hands are off, you flag work WILL look sloppy. On one handed stops, stopping strong is hard... your flag will have a tendency to move. Just work to develop your muscles so that you can hold your flag stationary with one hand.
Sails
First of all, sails are BAD! A sail is when the silk gets caught up on the top stopper, so your flag can't spin correctly. Sails do happen... and judges will understand this. However, judges (and your audience) will only understand for about a 5 second window. The idea is that you need to YANK your sail as soon as possible! You should never act like it's bothering you (keep smiling!), but you should casually pull on the silk to get it to come undone. If that doesn't work, wait until you have a .5 second rest where your flag isn't the air, so you can really YANK it. Also, do whatever you can to prevent a sail from happening in the first place. If there is spot where you KNOW you always get a sail, FIX IT! Sails attract the audience's attention... the sooner you can pull it out, the faster they'll stop looking at you.
Smooth it Out
Flag work is generally meant to be smooth and flowy. So, unless you're supposed to be doing something choppy on purpose, make sure that your flag is always moving... if you're getting to some check point early, and therefore having to stop, then start again, etc, you should slow your work down so that your flag can just move through the check point without stopping. Smoothness makes the show easier to watch and won't distract your audience's attention away from the group and on to you!
Pitch
Pitch is when your equipment spins outside of your "toaster"... the imaginary box that is around you. That means that your flag is spinning NOT parallel to the wall or floor (whichever it SHOULD be parallel to). Although judges can't always see this if you are facing them, if you are spinning facing the side, they will be able to tell. The bad thing about pitch is that your flag will hit you if you allow it to spin slanted... either in the forehead or the shins... and it will hurt! So use your muscles and keep your flag straight and parallel to your body.
Watch Poles Not Silks
The idea of colorguard is that everyone spins together... all flags should be in unison. Obviously, the way we accomplish this is by watching other people, to make sure that our flags line up. That's all well and good... as long as you are watching the pole in front of you, and not the silk. My guard director always used to say, "Watching the silk you can be off by this much [4 feet or so]. Watching the pole you can only be off by this much [an inch or so]!" and she was right. By making sure your poles are spinning together, you're guaranteed that your silks are together.
Free Hands/Release Hands
You should be aware of where your hands are (and especially where they're SUPPOSED to be) while you're spinning. If you are only doing something with one hand, most likely your free hand should be by your side... and not making weird gestures out to the side like it wants to be. That is something that just takes a lot of practice and really being conscious of. As for release hands... after you toss, usually you're supposed to leave your release hand exactly where you released until about the second before you catch your flag. However, you're going to want to bring it down to about your chest and wait to catch you flag... RESIST the urge! It makes much more of a statement for you to leave your hand, plus it generally makes it easier just to bring it down and catch your flag.