You may help control the warping a bit by painting the top and the bottom edges of the door. Most people never notice these surfaces, but in some doors a lot of moisture gets in and out through these surfaces, aggravating the warping. Other doors are just going to warp, so to maintain a good fit you need to adjust the catch on the door frame from season to season. I have fallen in love with a little piece of hardware that very efficiently solves this problem. It is a striker plate that has a stair-stepped inner piece that fits inside the hole in the door frame. As the tongue from the door latch goes into the striker plate, it ratchets up on the stair step. This means that the door stays tight if the tongue is on the first step, or on the last, but it is no longer banging back and forth inside the hole. You never need to adjust the striker plate, it does it for you all year long.