I almost always keg after cold crashing the secondary down to about 30-32. That way most of the yeast has dropped before kegging and the chilled, flat beer will take CO2 better.
I purge the keg with gas from the bottom up through the diptube, with the gas line connected to the black ball lock connector. Rack into the keg as usual (I use CO2 to start the syphon). Once the keg's full and the lid is on I purge the keg a couple times, again with the gas on the black ball lock feeding CO2 from the bottom of the keg, burping the gas from the lid vent and the gas-in sides of the keg.
With the gas at about 25 p.s.i. lay the keg on it's side (dip-tube side of the keg down) and gently rock it back and forth until you don't hear gas gurgling into the keg. Give it a minute and rock it back and forth some more. Usually takes about 20 minutes or so to quit taking on gas. After that, reconnect the gas to the gray ball lock like usual and stand it in the fridge overnight. Don't forget to lower the pressure before tapping a glass! I've done this as short as the morning of a big party and had cold carbonated beer that night to serve. Of course, 3-4 days in the keg at serving pressure is better if you have the time.