One way I've found to accelerate the process slightly is to turn the keg on its side (more surface area of beer exposed to the CO2) and rock it back and forth with the pressure on, for a good few minutes. Then stand it back up so beer doesn't work its way into the lines. If I do that a few times over an hour or two, right after kegging, I can normally get good carb levels within a week or so.
-Kevin