Digital files should be saved in the native format of whatever program was used to create them. In other words, if you made it in Photoshop, save it as a Photoshop file. If you made it in CorelDRAW, save it as a CorelDRAW file.
Vector files (Illustrator, CorelDRAW, Freehand, etc.)
Vector files translate into T-shirts better than any other file. Please design them at the actual size you want printed, and either include any fonts used on the CD, or convert fonts to outlines.
Raster files (Photoshop, Painter, etc.)
Raster files should be created at 300dpi at the actual size you want printed. Leave them in layers (unflattened,) and please include any fonts you've used on the CD.
Desktop publishing files (Word, Publisher, Printshop, etc.)
These programs really weren't designed for professional graphic design use, and were intended more for home printing. If you designed your T-shirt masterpiece in one of these programs, that's fine! Just print it out on paper with your home inkjet printer and bring it in, and we'll do the rest!
Please, please, please write down the fonts you used, and our artist will thank you for the hours of font matching you saved him!