Originally Posted by woer
I use the CPU Tuner app for that purpose. It has a ton of options--it's basically a GUI for cpufreq--but the relevant ones are you can shut off background sync and wifi and bluetooth, turn on airplane mode, set the min and max cpu frequencies, and set the cpufreq governor, all to whatever you want when the screen shuts off (you can also set all of this based on the battery percentage).
I have mine to a conservative governor and everything off but pulse on wifi and background sync (which means every 90 minutes wifi turns on for 5 minutes to allow apps to sync and then turns off again--actually even those ranges are customizable). With everything off and the max freq to 100mhz, my 4.0 only loses about 3% battery overnight.
Going back to this, yesterday I modified the "screen off" plan in CPU Tuner to turn on airplane mode (and modified each other plan to turn it off) and also set the max speed up to full (because mp3s skip with only 100mhz as the max). I also changed 100mhz to draw only 800mV in Kegrak.
I went to bed with 99% battery and woke up with 99% battery. That's right, not even a 1% drop. For saving power with the screen off, this is the way to go!