DO NOT, DO NOT, DO NOT squirt WD-40 into lock mechanisms!!! :shock: :-(
This is a sure-fire route to premature pin/tumbler death. Just because it solves the problem of the moment doesn't mean that it isn't setting you up for more serious problems two years down the road. The crud attracted by the residual lubricant in WD-40 is wearing down your key and the pins every time you lock/unlock.
This won't be nearly the issue with household locks because you aren't driving your house through road spray or dusty conditions. But even with household locks, mention doing this to any locksmith and you will get that scowl-over-the-reading-glasses look that says "What were you thinking?"
Use a lubricant designed for locks, available at any hardware department. These have a solvent (for cleaning) which quickly evaporates and leaves a dry lubricant which isn't an engraved invitation for dirt.