I just read about somebody who, without really turning the N800 off still had battery power after 8 days of no use. My N800 is empty after half a day if I do not turn it off.
What settings should I change in order to prevent my N800 to be completely empty in such a short time?

are you useing it for half of the day? It will last for well over a week but only if you let it alone for that week. You can lower the screen backlight but thats about it.

Also it depends a lot on what 3rd party apps are installed. Some might be badly broken and keep the processor busy, therefore preventing it from getting into low power state.

Lock touch screen and keys should help.

Unless there's something chewing CPU it should last for days, with normal on-off use (even with wi-fi connected). I've seen the occasional almost-empty battery in the morning though, one of those cases I traced to where I had opened an xterm, started the 'vi' editor, and then terminated the xterm without quitting vi. A check showed it looping.

Could Pidgin be doing this (keep the processor busy)?

Start an xterm, enter 'top' (without quotes).
You shouldn't see more than 1.3-1.7% cpu by Maemo-launcher (caused by xterm+top itself), and the occasional Xomap a bit higher. And Load average should be in the 0.05-0.10 area (for all three values). Quit top with 'q'.

Actually I think its the WLAN connection which I set to stop after 10 minutes, but which doesn't do so . I already stopped email and rss from automatically updating. But somehow WLAN must be stopped manually and does not disconnect after the pre-set 10 minutes. It should though .