Looking at other people's giveaways, they always have rules: Must be following, reblog/like only X amount of times daily...
also Tumblr's rules: www.tumblr.com/policy/en/contest-guidelines (I'm not sure about the over 18 one..I've seen some people under age limit participating, but I recommend sticking with 18+, since supporting minors giving away their addresses isn't good)