Ever done a giveaway?

So now that I am launching my official professional face painting business, I was thinking I would get the ball rolling with a Facebook giveaway for a free 1-hour birthday party. I haven't seen anything like that before (as far as the face painting part goes) and I wondered if anyone here has tried it.

So if you have tried this or something similar, was it successful and worthwhile?
Any words of caution?

Thanks everyone

I have gave a couple of times a gift card in exchange for professional pictures, It worked perfect the first time because they had a party on a week day for 5 girls close to my house and they tipped me.

Second time didn't work out because:
1. You need to specify that the party is for week days, limit the number of kids, and disclose a travel fee if applies.

Thanks artyfaces!

I ended up wording the giveaway this way:
http://flyinglaughter.com/grand-opening-giveaway-open/

I did my best to cross all my t's. I hope it pays off.