Be careful on cancelling too fast. Remember that some of these companies watch what you do in the first couple of days after you sign up. If we have everyone cancelling too fast, they have the right to either 1) not pay me for the signups, or 2) pull the offer really quick so that no one else can do it.
That's what happened with Romeo awhile back. They saw too many people cancelling so they put a bunch of restrictions in place (no cash back incentives, no credit if people cancelled in first 48 hours, etc).