I also charge on retainer. I set a dollar amount that makes messing with their stuff worth my time, and then they get to tell me what I'm supposed to do for them that month for those hours. If they don't talk to me, I still get paid. If they go over, then I get an extra hourly rate.
This is for a part-time gig. I charge a set amount per month up to a max number of hours worked. If I work 1 hour or 10 it doesnt matter. Usually I only work 2.5 hours a month hence the high hourly rate.