Rafflecopter gets another vote for me, I am running an iPad 3
Giveaway contest now and it has 18,000 entries. As others
have mentioned, make sure you clearly state who can enter, if you
aren't willing to ship a product Internationally or you have
special terms, then you must mention it.
Leverage a Facebook event and invite everyone who can
participate, this can drive a big boost. Other things to help
promote contests are giving entries for others
sharing/hosting/promoting, this is easily done with Rafflecopter
where they can co-host the contest widget giving you more
visibility for your contest. Currently my giveaway is hosted
on 40+ blogs.