Rit Sun Guard Laundry Treatment UV Protectant
Add it to the wash cycle, rinse and dry as usual. One treatment is good for more than 20 additional washings.

This stuff is great. $20 for 6 washer loads of clothes. It takes a regular T-Shirt up to the equalivalent of SPF 30 (a new T is normally about 5). You just wash and dry them once with the Rit, then wash normally after that, lasts through 20 loads, doesn't feel any different, doesn't change color at all.