Biggest thing I found that drained the battery like mad was when I left my wifi area and had the check for new wifi set to every 5 minutes - killed my battery FAST. Since I have changed that to every 30 - or disabled it altogether it lasts a lot longer.

Oh handy tip for the fm transmitter - put you N900 on the left side of the dashboard - seems to be the key for getting clear signal for me in at least 3 cars.