Force Carbonating: There seems to be great confusion about how to do this
properly, but the fact is that there is only one proper way to do it and it's easy if
you know how. Forget about shaking the keg with live pressure on. This doesn't
properly allow the CO2 to dissolve into the beer. It results in a flat beer that
foams all over the place. It can also aerate the beer, causing DMS ("sweetcorn"
flavors/aromas) or oxidation ("wet cardboard" flavors/aromas). The only way to
properly force carbonate the beer is to chill it down to below 40° F and to then
dial the regulator up to 20-30 psi and to leave live pressure on for 1-3 days. If
you cannot chill it, it will work but it will take more like a week than 2 or 3 days.
The lower the temperature and the higher the pressure, the faster the brew will
carbonate. You will have to play with it to get it just right. You can always bleed
any excess CO2 if you overcarbonate. A good technique is to try 25 psi for one
day, and then to test the carbonation level by serving. If you need more, crank
up the pressure again and go another day. For many ales, you will find that
about 2 days at 25 psi is plenty.
From this website
http://www.beercrafts.com/kegcarbon.html