I underclock mine since I'm not yet willing to pull the trigger on the leaked update. I don't have any concrete measurements of how much my battery life has increased but I no longer worry about running out of charge before I get home. That said, I still can't use it as heavily as I did my D1 but it is better than it was.
Here's my list of power saving tweaks:
1) Freeze bloatware
2) Underclock to 800mhz using SetCPU
3) Use SetCPU profiles to force 300mhz when the screen is off
4) Screebl app keeps screen on when I'm using it and kills it when I put the phone down
- Have to be careful to manually shut off screen before putting phone in pocket, though...
5) Tasker auto-starts GPS when I launch apps that use it then kills it when those apps close
I grabbed a generic extended battery (1800mAh, I think) that fits under the standard back for the most part. Better, but still thinking I'll pick up a moto battery with the larger back.
Meh. Battery life is still the biggest problem with the D3 but these things have made it bearable for days spent away from power.