When I first got my Lumia 800 I did some tweaks to improve battery life. I’ve just passed them on to a fellow owner, and they helped for him too, and so I thought I’d blog them (mainly so I don’t forget myself)
I turned off “Synchronise game requests” on the Game settings and “Notify me when new networks are found” from the WiFi settings. I also set “Download new content” to 30 minutes on both my email inboxes. I configured the Kindle application to display text as white on a black background, this makes a big difference to the consumption on the OLED display.
With these settings, which don’t cause me any inconvenience at all I can get a day of use out of the phone with enough left to limp through a second day on battery saver mode if I forget to charge the phone overnight.