Most of them, right after you get the confirmation emal (Welcome to ...) you can cancel right then and there and still get the points. Some of them you ahve to call and cancel and listen to them whine and try to convince you to stay, but just keep saying "I want to cancel, I want to cancel ...".
I was looking through my acct last night and realized I never got my points for trying out CompleteHome (did get my free Home Depot gift cards, though!). I need to contact MyPoints about that one. That was 200 points!!! The offer on the website right now is a free George Foreman grill and 200 points (trial is $1).
Got an email about signing up for one (think it was Buyser's Assurance) for 400 points. Didn't do that one, but I might change my mind if I get another email (think the regular link on the website is only 200 points).
I will say it was like pulling hen's teeth to cancel that Ancestry.com one, though. Took forever to reach anyone on the phone, and the guy would not let up!