Know what services you need before meeting with the vendor. Each call center has its own expertise, infrastructure and technology. It may have had experience on a different help desk support in the past. By knowing its major strengths, you can get a good deal if you let them handle several services.
Real help however begins with people who provide the service themselves by equipping them with tools and processes to make them effective front-liners. Once they do, this satisfaction stays with them and is passed on to real customers day after day.