Doors stick for a lot of reasons: a loose hinge, too much paint, a settling house, or humidity in the air," says TOH general contractor Tom Silva. If the door suddenly jams in dry weather, your problem is the hinge, likely a loose screw. A door that's been painted too much or is sticking because the house is settling can be sanded or planed at the trouble spot, then repainted.
If the door is swelled by humidity—often a front door or bathroom door—it's usually because part of it wasn't painted and moisture is getting into the wood; check the top and bottom edges. Shave down the door as little as possible to make it just fit. "Normally you want a crack the width of a nickel around a door, but in this case make it a dime," says Tom. Then paint over any exposed wood on all six sides to seal it. When the humidity lessens, the door will shrink to a perfect fit, opening without a hitch.
Get the full step-by-step instruction at How to Fix a Sticking Door