The Grunge - or - How to make a new shirt look old.

New shirts looking old is all the rage today. You can see it in every store. Target, Walmart, Old Navy, Aeropostale, etc. Achieving that look on your graphics is pretty easy. There are two general approaches. The easiest and most expensive (we always have to pay for convenience) are pre-made photoshop filters. MisterRetro offers some great image filters that can torture your image in an almost infinite number of ways. A cheaper but slightly more labor intensive way to add character to your graphic is using the standard filters offered by photoshop and combining them with textured art. Here are some links to online tutorials for distressing artwork:
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I have used both and I feel I have more control with the latter. Remember to take into account how the shirt will be printed and what color blank you will use. Some printing techniques used to print on dark shirts first put down a white layer before printing the colored layer. This will frequently lead to a less than desirable final result (a la CafePress).