I assume by dual battery it means you're using an N900, though you never specificed which phone you're using.

I would disable the automatic update checks, disable automatic email checks, install conky and it shows active network connections and up/down bandwidth activity. Disable automatic connection to gprs. Install autodisconnect to cut it off in case you forget and leave it connected. Don't use IM or VoIP. These are the things I can think of to minimize cellular network usage.