and be religious on replacing it on a regular basis b/c if you dont

we moved into a rental for 10 months between selling and moving, we've always changed our filters monthly so I set a control to do that at the rental but before a month had passed from our move in date we had a horrific period of heat and the a/c started not cooling as it should. landlord sent out HVAC folks and come to find out her previous tenant had NEVER changed the filter (in what they estimated was over a couple of years b/c it was the same filter the HVAC company had put in on a prior service call-they would initial and date them to monitor for subsequent calls). think of what happens to a straw when you try to drink a thick milkshake-how it collapses into itself-that's what the ductwork did. throughout the house all the ductwork had to be replaced and the HVAC was fried.
some people refer to the first day of each month as 'rabbit-rabbit day'...in our house it's 'change the filter day'