I saw something of a craft tv show, never tried it myself though. They sewed different colors of knit to the wrong side (3 or so?)and then cut through the the top layers leaving only one uncut, a X cut, maybe 2-3" across. When washed it curled up. The X's were either "randomly" placed some in a pattern.
**Another t-shirt idea:I helped with a activity at school. The kids all brought different colored T-shirts then collected leaves and laid them on the shirt and sprayed w/mister bottle a solution of bleach around the leaves(not sure what concentration) and let sit 10 min. or so then rinsed. It left really nice leaf patterns. You can do front and back but you must line in with cardboard wrapped in plastic bag so the front/back doesn't soak through. The best ones were made of 100% cotton. Sorry, can't remember if t-shirts were wet before bleach was sprayed on, I think they were.

Thank you Izzie, I really like the ideal with the leaves.

You can cut from the arm hole down each side. Put gromets on each side. Then lace up like shoe laces.

You can get wooden stampers at the craft store. Put paint on them, and stamp your t- shirts.
Dogs, cats, horses, bears etc.

I do about 10 different embellishments to my t - shirts.

Mary Mulari appears quite frequently on Sewing With Nancy and always with lots of ideas for T Shirts and such. I know she has a book out and you might find it first on the Nancys Notions site and then check with your library or local fabric store.