Build a board of advisors of three year olds and email valleywag. Make firebreathers wear tshirts to show how hot your start up is. Create disposable tattoo which doubles up as an invite code when put under the shower.
Email me for pointers, i love this sort of stuff.
Iron-on tattoos. Water-based temporary tattoos have been done. Iron-on ones would guarantee some sort of press coverage!
Seriously, though, wouldn't it depend on the nature of the audience you're trying to attract? Three-year-olds would be great for a preschool games site but not so good for, perhaps, helicopter pilots. Can you be more specific about your startup?