Use the same carrier for your whole trip. Staying with the same airline for all the legs of your trip tends to be much cheaper than using multiple airlines. Fly on Tuesday, Wednesday, or Saturday. These days are less popular with business travelers and are therefore cheaper. Book as early as possible.
Airlines tend to increase their prices as seats fill up.
The fewer seats available, the higher the price will be. Avoid traveling during peak holiday periods. Fares are higher during the holiday season, as more people travel at this time. To avoid the crowds, travel on the holiday itself such as Christmas Day or Thanksgiving Day. Check to see if you are eligible for any discounts.