I avoid those, you cant grab the creds then cancel your cash deposit, they also have a reputation for not coughing up the credits even after proving your case.
I've used free trials for tv subscriptions (amazon, nowtv etc.) just remember to cancel b4 trial runs out or you'll be charged. The creds get paid almost immediately and cancelling the subscription is simple. There are quite a few like this. Best to open a new e-mail account just for these offers, they come with newsletters and further offer e-mails.
Surveys can be hit or miss but pay well. Free draws for i-phones,supermarket vouchers etc. bring a lot of creds too, take a screenshot of page confirming you've been entered in the draw/offer, then if it doesnt pay you e-mail them the pic as proof, they pay out after that.
There's a report button on offers page to start a claim.
I very rarely put real money in here and I've had sh*t loads of equipment upgrades. Good Luck!