IMO, the best way to keep sub plans in a reasonable state of usage is to NOT allow people to do no strings attached one off months. Like the stories you hear of people signing up for a month, plowing through games, then stop. Then repeat when the next hot game comes out in 6 months.
When someone does a cellphone or cable tv plan, it's pretty hard and stupid for anyone doing a temp one month plan because the company will likely charge you activation fees, and/or charge you the price of the cable box (my indie provider charged me $100 to buy out the box since they dont rent them out for a monthly fee). So when you got this much invested in it up front, nobody is going to bail after 3 weeks to test it out. You'll commit and stick around.
But a lot of tech, music, gaming, movie plans allow people to zip in and out when they feel like it with no limits or annoying upfront fees. I remember a recent thread where someone said if you sign up and cancel an Xbox sub plan in the same month they even refund you the monthly fee. Is this true? If so, that's even worse.