Nah, it's not even that difficult. Just call at night (like after 9pm) when you ask to have it added on, then do the same (after 9pm) when you want to take it off. Do it after 9 because that's well after the "Retention" department is closed (the people who will ask you "Why didn't you like it? Would you like to try a different service? How about if we discounted it?) and the Repair Department (me) will be happy to take the services off your account quickly and with no questions in order to lower our Average Handle Time with a nice 2-minute phone call. The addition and/or subtraction of services, thanks to the Digital cabe box, is almost instantaneous.

Now, if you feel a bit uncomfortable with this, you might want to call them up and play all innocent with your questions. "I'm thinking about adding HBO to my account, but what do I do if I don't like it? Can I jut cancel it at any time? Will I be charged for a whole month?" Don't be confrontational; avoid sayng "Well this guy on the internet told me I could get it for just a day so that's all I want to do is just one day".

Also, don't do it more than like once every few months or so. And do lots of swapping, too. Keep HBo fr a week, then call and say you want to try Showtime out. Then you want to try Starz, then Cinemax, etc. You never know when the rules might change, and having an actual history of indecisiveness will help to cover your tracks.