[whpalmer4]

I put them in OmniFocus, with a start date a few days in advance, and a due date. I also use a "tickler" perspective every day which shows me all the actions that started that day. That gives me the initial signal, looking at my due soon list gives another.

If you wanted to be a bit more thorough, you could set up a repeating action group or project that tracked receiving the bill as well as paying it.