Soak your feet in vinegar. Sounds weird, but the acid in vinegar helps dissolve the fiberglass particles.

***edit for new info** Ok, don't pay attention to my 'I'd probably soak 'em in as-hot-as-you-can-stand water first for a few minutes to open up all your pores' because all the things I just looked up said this would only encourage the fiberglass fibers to work their way further into the skin. Tepid water is best, followed by the vinegar, & when your feet are thoroughly dried, dry brush them gently because as your skin exfoliates, it encourages the fibers to come out on their own as well.