I've been using my Blackberry 9700 for about 10 months now and am loving the phone - but a lot of collegues are complaining about battery drain in 16 hours, and general poor battery performance. As with any smartphone, the choice of features dictates power consumption, and here I've given a short breakdown of what to change/disable:
3G Mode - With most BIS implementations, you're only getting data at a maximum of 300kbps for UNCACHED local data, nowhere close to 3G speeds currently available, rather throttle back to EDGE (238kbps) and save on the transciever wasting power jumping between 3G and 2G mode (Manage Connections/Mobile Network Options/Network Mode=2G). Granted there will be a speed difference, but if you're using the Blackberry primarily for mail and not as a web browser, you should not notice any difference.
Bluetooth - If you're not going to pair with a headpiece or a carkit on a daily basis, rather just turn this off - even without pairing the battery use from constant device polling reduces efficiency (Manage Connections/Tick "Bluetooth" Off)
Wifi - Another bugbear - WiFi is great for high speed downloads but inside the office, unless you're using a dock to re-charge its not really worth it. Also even if you are not signed into a WLAN, the Wifi transceiver is also still searching for SSID's periodically, using valuable power. (Manage Connections/Tick "Wifi" Off)
GPS - Unless you're using the GPS daily - switch it off, even location based assistance will constantly be in data contact with Cell towers, using battery indescriminately (Options/Avanced Options/GPS/GPS Services "Location Off"; Location Data "Disabled"; Location Aiding "Disabled")
Standby - Switch the phone into standby with the little "Lock" button at the top left of the phone, this will shut off the screen after a few seconds and idle quite nicely. BIS E-mails will still be delivered and the phone will still ring when a call comes in. A simpler way is to use the leather wallet that came with the phone - there is a small magnet in the bottom that signals the phone to go into this mode, and disables standby when its removed from that pouch.
With all these options enabled and a good 10-20 calls per day, the minimum my battery lasts before the phone dies is 6 days - I've had up to 10 days with reduced call volume. This set of disablements does not affect me at all, and the increased battery life is worth having them off.