Just remember to Tweet about it on Twitter, send out emails, or better yet, announce it in your monthly newsletter, on your website and blog. Done.
There’s a cool site called RaffleCopter that will help take the guess work out of creating a giveaway and picking a winner. On the flipside you can create a list in MailChimp with an entry form and gather winners or if you’re a Google Docs kind of person – create a form in Google Docs to share in your newsletter, on your website and blog and in your Facebook and Twitter posts. Once the contest ends you find an online number generator to randomly pick the winner(s).