Get fans to your gig and sell merch at the same time.

Getting in free to a concert is good news for any fan. That all the fan would have to do is buy a Fred Deakin (Lemon Jelly) illustrated t-shirt and wear it to the concert was brilliant marketing on Deakin's part.
TimesOnline on Fred Deakin

Deakin's a good marketer as well as a great illustrator, creating a win-win for both fan and band.

Now, if Lemon Jelly were looking for more exposure/press/fans, having a t-shirt signing party a week to 3 days before the concert at the local mall/park/college bookstore/wherever would have increased both t-shirt sales and concert attendance. Freebies are good news for family and friends and a lot share any they hear of through email and social networks like Twitter and Facebook as well as over the water cooler @ work. Word of mouth. About your band. Who woulda thunk it? :-)

As a final recommendation, if you have the capabilities, make a 'special edition' t-shirt with a really cool graphic of some facet of the concert location and/or your logo. You don't have to be or employ Fred Deaken. Lots of good free graphics programs out there list of great logo design tutorials. All of these can be converted to a t-shirt graphic easily. If you need help w/that part, let me know in the comments and I'll find you a site and/or someone to help.

Make your design simple, eye catching and easily read from a distance (think of a concert attendee walking down the sidewalk facing a future fan). You don't want a 'one time only' wear from the t-shirt purchase. Every time someone wears that shirt, it's free advertising for your music. A few years from now I want to see your logo on some worn and holey t-shirts at the laundry mat.
Molly