I built a little pressure gauge to help monitor my kegs during carbonation. I chill the beer down then hit the keg with CO2. I generally crank the pressure up as high as it will go. This makes sure the keg seals as well. I then check the pressure gauge every 12 hours. I hit the keg with CO2 if needed. I stop adding CO2 when the pressure stabilizes at my desired PSI for the specific temperature. I generally only have to add CO2 twice over a 24 hour period if the keg is sealed well. The beer always comes out carbonated perfectly.
Here is the gauge I made.

This.^Yes, a great solution. Very nice indeed.
I always am concerned with
over-carbonation rather than under...I
hate overly fizzy beer (and overly chilled beer) in general. I'll generally connect a finished brew at 30 PSI for a few days and start monitoring after that until it gets to the tickle I like.
If I decide to bottle some off, I'll get it to just under freezing, and goose it with a bit more bubble than I usually prefer to allow for what may be lost in the bottling process going into freezer stored bottles. That way I manage to deliver slightly higher carbonated bottles to give to friends who might expect a bit more bubble in their brew. If I need to, I'll bleed off the excess afterwards and atabilize it to the levels I like.
Gotta say though...may have to build me one of these...