My personal rules for making shirts.
1. Don't overdo the design. Something simple. 4 (or less) colors that stand out but match each other and the shirt is probably the best.
2. No incredibly massive designs...especially on the front. IMO, front images look horrible...sometimes on certain people more than others. For (almost) all people, the back is straight and flat. The front...well...that's another story.
3. Designs shouldn't go higher or wider than the shoulder blades. That's just personal preference, but so is everything else I've said
Starting too high or going too wide looks unprofessional normally. There are certainly exceptions to this, but not too many.
4. Make it large enough so it can be read/see without strain. OK. You're in a crowd of 10,000 kids and trying to find 40 other kids that look like you. Make something noticeable. I noticed many many many designs at nationals that all looked alike. I recommend a flashy design on the back and the team number/name on the front.
5. Use an original type of shirt. Or at least not a regular tshirt. Dickies shirts were really nice...one word: pocket-protecter capable!
(Heh...I can't count) MOE's shirts really stood out. Maybe it was the green...but the type of shirt was nice. Might be expensive, though.
So much for my 'rules'
I usually keep that in mind when thinking of shirt ideas.