Every time this "do-we-exist/prove-it" question shows up on the Internet somewhere (and it shows up over and over and over again), my response is always to bring up the row-of-the-saltines point:
If I take out a brand new row of saltine crackers, open the wrapper, and proceed to eat the whole thing (which I don't really do, by the way - only half the row! ); a couple of hours later there will be the torn open wrapper, and no crackers left. That, to me, pretty much lets someone know I'm not a figment of anyone else's imagination.
I suppose if someone were to get into forensics, there would be traces of saltines somewhere that connected me to the saltines.
If someone tries to take away my saltines, and I punch him in the nose, give him a nose-bleed, and leave the imprint of my ring and fingernails on his face, I'd say that pretty much proves I exist too.
Except for those people that get into the whole, "How do we know what's real is actually reality" thing, nobody asks me to prove I exist (although a lot of people act as if I don't, sometimes ). Still, I'm here, potentially punching noses if I have reason to - so if anyone asked me to prove I exist I wouldn't find it difficult.