Texture

Just like in cooking, texture is an important part of t-shirt art. I guess we can say that texture itself is an important part of any art. In Photoshop, you can achieve texture in a variety of ways. I have been experimenting with texture brushes. I used three different, custom brushes to create the following margle-like texture:
Here are the Photoshop brushes I created and used to do the above:

Three Cool Brushes

Great Shirt Graphic Tutorial

Some great tips regarding design and t-shirt production. Requires a pdf download.
Photoshop t-shirt design tutorial

Useful Dingbats

Here's a site I ran across that gives some great dingbat graphics that would very useful given out recent discussion on grunge:

Useful and Free Dingbat Fonts