I used cafepress to make some stickers. The stickers they have available are all too big, so my solution was to print several copies of sticker images on each sticker, then you can cut each out to use or share. This way, you get more stickers for your money as well. If you'd like to see how they came out, you can see them here:

http://bike.bedope.com/

I also had an idea involving Einstein: I drew a picture of him and used the phrase "Geniuses Ride Bikes".