You never know what someone's financial situation is. Especially with something like a rent check - for many people, that is their biggest expense. You could try dropping a hint. If that doesn't work, you could get a money order or bank check each month - kind of a pain, but at least you wouldn't have to worry about it.
Another option I have with my bank is using their online bill pay. If I set up an individual on Bill Pay, it will mail them a check and immediately deduct the funds out of my account - doesn't matter when they cash it. You could look into that.