Keep the yard neat, the shrubs trimmed, and the lawn cut.
First impressions are important.
Keep the house tidy - not as a furniture store display window, but as a comfortable home to live in. Also, have the garage clean and neat. If you have pets, make sure their living areas are spotless and odor free. If you have a dog, keep him out of the house and under control during showings. Make all minor repairs, such as sticking doors, broken window panes, loose door knobs, plumbing, broken light switches, etc.