Here's what I do to force carb. Chill it first, then hook it up to the gas in. I'll crank up the pressure to about 30-35psi and lay it on its side (for more surface area), then roll it back and forth for a few minutes on the ground. Beats having to man-handle it to shake it!
After a few minutes of rolling it back and forth, I disconnect it, purge the pressure and turn the regulator down, then reconnect the gas to test out the carbonation.
If I get a decent amount of head on the beer, I'll let it sit overnight and usually it's pretty close with smaller bubbles by the next day. If it needs more, I'll just do the same thing again but only for about a minute or so at a time.
Note that I do use pin-lock cornies which don't generally have pull tabs on the safety valve to purge, so it does require that I remove the gas line to do it. One thing you do want to watch out for though is turning down your regulator before disconnecting from the keg and dropping the pressure in the keg. Even with check valves installed, I did manage to get beer in my Co2 line when I wasn't paying attention and turned down the regulator first.
If for some reason I manage to overcarb it, which I did do a couple of times when I first started experimenting with this technique. I'll just disconnect the gas line, purge the pressure, shake it, purge it, shake it, purge it, etc until it gets better (usually doesn't take too long).
It may not be an exact science of getting the exact volume of Co2 in the beer that each style should have. But if you set the serving pressure where it should be for that style, it usually equalizes fairly quick. Or at least a lot faster than waiting for it to carb over time.
But then again, if you have the patience to hook it up and forget it, by all means do so!
I personally wasn't born with much patience