I don't know how many of my professional friends here use t-shirts to advertise, however I just found a good deal. Not sure of the quality yet as I am waiting for my first order. They would also be good for "stage crews" if you use them.
I have done t-shirts from zazzle.com in the past, but they are expensive. I don't do "runs" as I have no need for that many. However, my daughter, husband, and me will wear a t-shirt advertising her website from time-to-time when the situation warrants it. (i.e. before a performance setting up, a day at the park where there tend to be families, etc.)
I have used Overnightprints for a while now for business cards and postcards. Just the other day I received an email that they were now doing t-shirts. One sided will run you $12.95 and double sided just $17.95. For small runs, trust me, that is a good price! Also sign up for their subscriptions as they routinely send out coupons. I got $10.00 off my first order.
We did a design in the front in the upper left hand corner that just says her website, on the back we have her name boldly strewn with a colorful caricature of her underneath.
I will let you know how the quality looks once I receive it.
Take care my friends!