Find individuals that fit the location and time period. If you are doing a movie about 2nd-century China, you don't want to cast Caucasian extras. Do research about the time period and location of the movie to make sure you find individuals that should actually be in the background of each scene (without confusing individuals watching the film).
Cast individuals who have an open schedule. Because extras are expendable, it is extremely important for these individuals to have an open schedule. You can't schedule your shoot around an extra, so make sure the extra can schedule around your shoot.
Look for individuals with an upbeat attitude and who are looking to have fun while working. Often these individuals are sent off in groups, so they also need to be able to follow basic instructions.
Ask about any pets or cars that extras own. If you are doing a period piece, an individual may already own a car from the time period. It is possible to obtain the movie extra and his vehicle for far less than it costs to locate and rent a different car. The same is true with pets, as on most city streets there is someone walking a pet in real life. Renting a trained animal with an animal handler is incredibly expensive, compared to the price of finding a movie extra with her own animal.