I've only done one giveaway on my blog, but I posted it here (obviously), put it in the promotion teams, Tweeted it, and I'm pretty sure I mentioned it on Facebook. You could also keep an eye out for linky parties that might apply.
I actually emailed a bunch of my blog friends with a graphic and note about the giveaway asking that they give it a little plug on their blogs. They all didn't do it but some of them did. :)