A cover letter reminds the Tenant of the date of their last adjustment, and of course mentions rising insurance and other costs, and "we hope you choose to remain".

I'd remind them, that you don't charge a premium because you know certain things aren't as perfect as they should be, ie: door knob.. and you think they may be happier with a higher end, more expensive home." --- something along those lines, remind them they are paying less in exchange for a few flaws-- if they want perfect-- go pay for it.   

Also, most groups (if not all) are once a month so put them on your calendar, set reminders, and attend!!

Send them something that reminded you of them out of the blue, buy them a coffee on your way to showing houses with them.

Thanks for the reminder!

Remind them that if they do have a pet, they may face a fine or termination.

Steve's comment reminded me of something similar that occurred Saturday at an REI Conf.

Remind her she has a deposit for this unit (which you hopefully got from the previous owner). 

I would keep it simple and remind him that he has been told hopefully in writing not to do this yet he did it again.  

Remind them not to use dish soap in the dishwasher, etc.One thing you could do if you are renting by the room is include weekly cleaning of the common areas as part of the rent.