I have one thing to say about "specialized" software packages. Unless they are professional they suck. You should learn something with alot of power such as Adobe Photoshop CS and Adobe ImageReady CS and create a custom template for your T-Shirts. There are many cool effects such as layers, alpha channels, blending options, filters, etc. Having those types of tools should make your output render some cool results. Just calebrate the software so it works correctly with whatever your are using to print the shirts. If you are using basic iron-on material, a regualar image file should work for you. If you have to print in layers you can create your image in different layers on photoshop and print each layer independantly. They use this method with silk screening. Anyways, hope that gives you some ideas.