Check with nearby universities and museums to see if they sponsor programs where volunteers can help on an archaeological dig site. You may have to enroll in a class at the university or pay a fee to participate in the dig.
Check with the U.S. Department of Agriculture's Forest Service Passport in Time program. The program offers archaeological opportunities within the Forest Service's parks. The jobs range from two days to two weeks in length. There's no cost to participate but you are responsible for your own travel and sometimes your own food.
Decide on a location or a time period of interest and search for digs related to those topics. You can find a good list of digs on the Archeolink website. (See Resources).
Check out the requirements and accommodations. You don't want to go expecting to stay in a hotel only to find you will be sleeping in a tent.
Choose the dig that best fits your location requirements, time frame specifications and budget.
Know what you need to take with you. The dig organizers will give you a list of the proper equipment. You may be living in tents on site, or backpacking in to the wilderness, or you could be in a hotel, if the site is near towns. Some programs include food, some require you to bring your own.
Be ready to take instruction from the experts at the site. As a volunteer, you will be expected to follow directions. After all, you may be dealing with priceless artifacts.
