If you can choose when to sell, it's best to wait until your local real estate market is "hot," or a "sellers' market." This can occur on a very local basis, regardless of what's common across the United States. The heat of a market can usually be gauged numerically, because the number of available homes drops well below the number of buyers wanting them. Here are some other indicators that the market is good for sellers:
Of course, not everyone gets to choose when to sell. If the market is down and you have to move immediately -- for example, because of financial reasons, a divorce, a job move, or an imperative health concern -- and you don't have any of the advantages listed above, you may have to settle for a lower price in order to make a quick sale.