Ive had to cancel tons of trials and lemme tell you, you HAVE to be firm.
For the medical ones(weight loss things) tell them your doctor advised against it and you have a medical condition where it could kill you !
For everything else just say you were offered a job over seas or your a soldier or something. The bottom line is you have to be FIRM, give them no chance to get you to stay, and ALWAYS get a confirmation number. if they dont automatically give you one, ask for one.
Also, NEVER tell them you just did it for the free games, you dont wanna mess it up for the rest of us!