At an xterm prompt:
1. sudo gainroot
2. cd .kernel
3. kernel-config load ideal
4. kernel-config save profile1
5. vim profile1
Press "i" to get into insert mode where you can edit, press "esc" soft key to get out of that. type ":wq" to save an exit, and ":q!" to exit -without- saving. ":q" will exit if you haven't made any changes, or alert you if you have.
6. Starting with 600mhz and working up, type(for each frequency):
a. kernel config lock *freq* *volt* *dsp*
These settings are the ones in that file I just had you edit, in the form "*freq*:*volt*,*dsp*". You want to first use the same settings you got from that file(for whatever frequency you are testing), then work your way down(more about this later).
b. "mplayer /path/to/short-video-file.avi"
You want to run some sort of heavy video file, something that taxes the N900 the whole way. I used the flv file from This music video. Even at 1150mhz, it still drops frames(its overloading the CPU).
When you play the video, one of three things will happen: 1, the N900 will crash instantly, 2, the video will have errors in it, stop entirely for a couple seconds or other issues. The console will say something about decoding errors. 3, everything will play fine.
If 1 or 2 happens, you have reached as far down as you can go. Now, take whatever volt value you had, add 2 to it, and stick that in the "profile1" file in the correct spot. after exiting the file, type "sync" to make sure it gets written to the disk. Repeat with the next frequency.
If 3 happens, go back to the kernel-config lock line, and subtract 1 from the voltage number you used, then try again.

Once you are done with all frequencies(aside from 0mhz), type "kernel-config load profile1" to load it. After you are sure its stable, you can type "kernel-config default profile1", but be warned that I've had poor luck getting boot videos to play correctly, even though everything else does.