The first thing you need, as with any location, is as much correct information as possible on the subject you're trying to locate. The more identifiers, the better your chances for success. For instance, if you're trying to locate a person with the surname of Williams, and that's all you have, your client needs to have a lot of money, because it's going to take a lot of time. There are a lot of Williams' scattered across the fruited plain, so you need to reduce the size of the list. I like to have three identifiers, A complete name, a birth date, and a social security number.
Time can be a factor, but that's one of the two things required to do the locating.
It's not always possible to have this information at the beginning, but shortly after I start looking, I'll have it. If your client can only provide you with a name, and an old address, you can access the credit bureau computer data, obtain a credit header, cross reference the name with the address, and obtain the subject's social security number, and a list of the subject's previous addresses. You can then run scans on the neighborhood, and learn everyone's name, address and phone number that lived or lives next door, across the street, and around the corner from the subject. Here are some data base scans you should begin with. You'll quit running the various scans when you retrieve the information you need.