Increasing the talk time of your smartphone

First of all, if you go out on a field trip or camping, make sure you take a USB charger :-)

Here are some tips to conserve your Android phone battery:

• Set the display backlight to minimum
• Disable automatic syncing
• Turn off bluetooth, GPS and wifi

(you can use a widget to access these settings quickly)

In an emergency situation you might want turn the phone off and turn it on every hour just to check your messages.

A second battery is another option to consider or you can get an accumulator that can charge any phone with a USB plug. I have a powermonkey explorer, but unfortunately it died. 

other links:
How to Keep Your Smartphone Battery Going - Wired
Extending smartphone battery - Chronicle