My wife and I are starting a travel agency (choicetraveltx.com) and we are looking for ways to build our business. One thing that we have discussed is having a contest featuring some substantial prizes that would be of interest to people who like to take cruises (since that is our favorite form of vacation, we decided to specialize in cruises). So far, so good, we have a budget of about $500 for prizes, and some ideas for how to award them.

Problem is, we don't know how to effectively track the actions we want. The initial contest would be something on Twitter, where a person would enter the contest by retweeting a specific message, signing up for our newsletter, and (optionally) making a comment on our blog. I've read that just using Twitter's interface does not do an adequate job of tracking retweets, and I'd really like to automate that process as much as possible, in order to build a list.

What sorts of tools are available for managing this sort of contest? How much do they cost? How do you effectively track the effectiveness of a contest?