CorelDraw up to v. 13
Adobe Illustrator up to v. CS2

In general, keep in mind that most screen printing is a spot color medium, as opposed to offset, where the majority of color work is process color.

Do not apply any trapping to the image.

If identical objects overlay one another precisely, please separate by layer.

If appropriate, submit files with all colors converted to spot colors, and all unused colors deleted from the document.

Unless there is a large body of text in the file, please convert all type to outlines or curves. As most t-shirt designs are not text intensive, this will prevent confusion and save you the time of loading fonts onto the disk.

Use the Pantone Matching System for spot colors. Be sure to check the PMS book for color accuracy as the on-screen versions are often misleading.