In general, a company can best utilize a staffing service by taking advantage of everything that a staffing service offers. Many staffing services will provide basic training, sift through job applicants for you, and do the accounting legwork to pay the salary of temporary employees. The more a staffing service does, the less the company has to do, so it is important to find a staffing service you can trust and then let it do what it does best.
With this in mind, it is important to think through which staffing service your company will use. Contact other companies and see what agencies they have used in the past. Ask questions of staffing services regarding the size and skill level of their employee pool, as well as what kind of training they provide and who some of their clients are. The best way to learn about a staffing service, though, is to try it out. Hire a few employees through an agency, and see if you are satisfied with the kind of employees you are sent.
Help the staffing service by communicating your expectations clearly. What kind of skills and experience do you want your temporary employees to have? What time commitment and salary will you offer this employee? Do you have any other specific requirements for your employees that you want to communicate up front? The more information you can give your staffing service, the more likely it is that they will send you a satisfactory employee.