Steps to Force Carbonate your Beer: CO2 dissolves into beer much more easily when the beer is cold, so ideally, place keg with gas line attached into fridge and leave under pressure for about 24 hours. Adjust pressure down to 20 PSI for 24 hours.
The main detractor in kegging too early is the amount of yeast solids your keg will have. That's not a big deal to me. I actually keg all of my lagers early so that the keg will be spunded.