Note monthly class payments in your calendar
Here's how Kirsten remembers those pesky class fees:
Now that my daughter participates in a couple of activities (dance and piano) that require monthly payments, I've started writing the amount due on the calendar for the first class of the month (ex: T-dance-$34) That way I don't forget how much it is, and remember to send a check with her on that day.
I've also discovered the miracle that is online bill-pay. You can schedule recurring payments, and your bank will send a check each month. Just specify the amount, the recurrence, and the end date, and you never have to think about it again. This is how I pay preschool tuition -- it's lovely and mindless.