I have a very quick topic for today... T-SHIRTS!
The short form is, don't! Do not spend money on them. I can't tell you how many cons I've seen sink financially because they printed t-shirts they couldn't sell.
If you really just have to have t-shirts, try getting a dealer to come in and sell them. There are a number of t-shirt vendors who will sell your shirts, if you provide the artwork and a table in the dealer room.
The main problem with t-shirts is the overhead. It costs money to make them and you are unlikely to sell enough to recover the expense. I'm not saying cons haven't made money on t-shirts, it's just really risky.
And, oh yeah, Happy 4th of July!