Stay over a Saturday night if flying round-trip. Buy more than 21 days in advance. Pick a flight with plenty of seats left. Currently, it is often less expensive to fly on Tuesday, Wednesday, or Saturday. Avoid holidays, but flying on the day of a major holiday can be a way around poor availability and expensive fares.
Lower fares can make a few extra driving miles well worth it.
Consider frequent flyer programs and make sure you are registered with the frequent flyer program of any airline you fly.