Pollack suggests using ground meat and grated vegetables in the beginning because they're easier to digest for a dog who's not used to eating fresh food. Mayer advises using just one animal protein source at a time, since animal proteins are the most common allergic antigens. One protein source is also easier on the digestive tract. The most commonly used meats are beef, chicken, turkey or lamb to start. She recommends avoiding pork, since pork tends to be more susceptible to parasites.
Organ meats are also a good choice, but use caution when feeding liver.
If you feed liver, make sure it's from a clean, preferably organic, source. After choosing the protein source, add vegetables. Dark green veggies and carrots are excellent choices. Some fruit is good too, particularly grapes and dark berries. Be sure to wash them to remove pesticides, dirt and other junk - if organic produce is too expensive or unavailable in your area, invest in a good produce wash like Healthy Harvest to get the surface pesticides off.