Another trick - keep one of four screens empty and keep phone on it while you don't use foreca, facebook etc. Because then you unlock the phone any widget on active screen starts working and some of them (like foreca) communicates via Internet and that consumes your battery.

EDIT: and if you use magnetic case (like me) then magnet may occasionaly switch the keyboard switcher while N900 is in case and phone think that you open KBD.