I am currently running mostly stock software on my Nokia N900.
The relavent non-stock packages I am running are:
2G/3G selector widget
BackupMenu/BootMenu
OpenSSH Server
rootsh
my MCE clone
my Cell Broadcast SMS operator name widget replacement

Are there any software enhancements that I can install that will improve battery life? Will installing CSSU improve my battery life? Will installing a new kernel or BME replacement improve my battery life?

I keep my phone in 2G mode most of the time (unless I am doing heavy data usage then I switch to 3G with the 2G-3G selector widget). Does doing that help the battery life? (or would I be better off keeping it in 3G?) What about if I am passing through areas of low signal strength? (the train I ride on often passes through areas where my 2G signal will drop out completly or get very weak, would keeping it in 3G in those cases be better?)

Anything else I can do to get more battery life out of this thing?

get a new 1500 mah battrey reflash your n900
worked nice for me

For a number of years I've been also keeping the phone in 2G and only switching to 3G occasionally. Since a few weeks I have it always in 3G mode and the battery life has not changed significantly. I guess it depends on how often it needs to switch and reconnect so YMMV.

I suggest you run powertop and see if anything looks out of the ordinary. Also battery-eye (a python daemon with a GUI) can show pretty nicely what's going on (note: the daemon itself uses almost no CPU/battery).

My N900 with 3G and a few daily hours of mobile data and WLAN (but admittedly light use, mostly ssh here and there) has an average use of 21.9mA.

Now that I look at it, before I started using 3G-only and installed CSSU Thumb I remember being on 15mA. Might be the 3G effect (I can't imagine why CSSU/thumb would have any effect on battery life, unless the CPU does slightly more work when running thumb instructions).

For ~1200mAh 15mA means ~3days and 22mA means ~2days. Good enough for me

Where do you get the mA figures from?

In battery-eye, go to statistics and see "discharge current (mAh/hour)". I took the "average use" (using all data).

Try this:

https://wiki.maemo.org/Overclocking

at the end of this page are some of the useful scripts, you can make your n900 to under-clock when it's locked and to over-clock when it's unlocked. This will save some power

Also you should use SmartReflex function (automatic undervolting). This will do the trick to, specially when combined with under-clock.

You can also try to change POWERSAVE parameter in kernel settings to at least 500, or changing CPU Policy (governor) from ONDEMAND to POWERSAVE, it will fight to save power and extend battery life, but it wil for sure **** with tablet performance.

If you don't know how to do some of those, write, I will post more detailed answer.

It really depends of many factors, I cant go with 3g all the time, because Im always connected to internet (Because of being chat-online). If running on 3g my phone will die in 3-5 hours, but when connected to 2g network and running with tricks in my previous post it will last a day, which is all I need.

But yeah, when only connected to 3g without internet connection battery life do not change significantly.