easiest sure-fire way is to use a diffusion stone. it will carb to the desired level in hours, with no shaking or wondering just exactly how much co2 went into solution.
just so you know though, just because you carb quickly doesn't mean that the beer is necessarily ready for consumption. that's something nice about bottling, you can't really drink it until it is carbed, and that takes time, so the beer is allowed to age more.
also, remember, everything is system dependent, so what i do may not be the best for what you do. you need to determine what volume of co2 you want in your beers, what temperature you want to serve at, and that will tell you what pressure to keep the beers at. remember that you also need to balance the beerline according to the pressure, use 3/16 ID smooth bore thick walled beverage grade tubing, and it will give about a 2 psi drop per foot.
so for my system, i keep my kegs at 40F, use 10 psi, which gives about 2.3 volumes of co2, and i use about 5 ft of line to the shanks/faucets. this gives a good pour for me. your system will most likely be different, and you will need to make adjustments to it yourself.