Sorry to be so commercial, but Spring's around the corner, so check out all the t-shirt designs/mugs etc. available at my Zazzle store here (order for yourself, significant others, or for conference attendees!):
Some of the current designs:
"Prime Numbers ROCK!"
"Math Is Infinitely Cool!"
"Euler, Gauss, Hilbert, Riemann ...The Fab Four"
"Like Math...? Let's Frolic!!""Show Me Your Algorithm and I'll Show You Mine"
"Viva La Math!""Math Is Sixy!"
"WWMGT (What would Martin Gardner think?)"
"Math Sizzzles ...and so do i "
"Revenge of the Math Geeks"
"M.I.B.T. (Math iz a bee-eautiful thang!)""Science Buff or... Buff Scientist!"
"1,2,3,4,5... Yo! What's your Erdos Number?"
"Blogito Ergo Sum (I blog therefore I am!)"...and more!
--- please note that I've generally posted the most inexpensive styles Zazzle offers to hold costs down, but you can easily choose to order any design in a different color/style (than the one shown) and pay slightly more.
...and if you buy all these up... hey, we'll make MORE! ;-)