Many fee-based genealogy websites have annual and monthly subscription plans. While usually an annual plan is most economical in the long run, it may be that if you do your research in "fits" and "starts" that a monthly plan, activated when you know you will use it, may be the best bargain for you.
No matter which plan you choose, know when your renewal is up and when your card is going to be automatically charged. Set a reminder for yourself at least three days before that date so you can decide if you are renewing or not. That should give you time to contact customer service if you wish to change your plan.