I got a press release this morning from the BlueCotton t-shirt company about a survey they did this month about t-shirts (natch). Specifically, the type of t-shirt people hold onto forever. Turns out that, while the top response was t-shirts people got while on vacation (48 percent), the next highest response (30 percent ) went to t-shirts they got from a business or employer--and I'd wager most of those are related to events or conferences.
I know some of my all-time favorites have come from conferences I've attended. One I got from a Religious Conference Management Association conference I went to a while back I wore until it shredded and my husband made me throw it out. One of my new faves is one I got at last year's ASAE and The Center conference, a screaming yellow number with "my editor thinks I'm on assignment" on the back. It seldom fails to be a conversation starter, and I just loved that someone thought to do that just for us press people. Though I do still hang onto a Grateful Dead t I picked up back in the day, even though it's way too small for me nowadays (concert t's came in third on the survey).
So, while they may not be a new idea, I'd say that t-shirt giveaways, particularly if they're clever/cool/specifically made for your audience, will have long shelf lives (or is that off-the-shelf lives?) for your participants. We may chuck out the conference bags, but the t-shirts will stay with us forever.