1. Lower your LCD brightness - this is the biggest drain, try to set it as low as you can without losing the ability to read the screen. [tip: if you are done with your phone, hit the power button to turn off LCD, down't wait for the 10 seconds to turn itself off... this is a big energy saving tip!]
2. Reduce LCD timer - change the auto LCD dimming from 1 minute to 10 seconds, this is just as important as the LCD brightness.
3. Lower your ring tone volume - any sound made through the loud speaker(s) will drain power, also, avoid using the loud speaker during calls
4. Turn off vibrate when ring tone sound is on - the vibration motor drains the battery
5. Turn off push-email, email sync'ing or drag out the server checking to a longer duration, instead of every 10 minutes, make it every 30minutes or 45minutes (make it manual if you are not expecting anything urgent)
6. For 3G devices, change network setting to GSM mode only. 3G radio drains battery much more than GSM/EDGE network. If you must, turn it on when need to.
7. If necessary and possible, set it to GPRS mode for data only. While GPRS is unbearable to surf the web, it will reserve more battery life, like the last option, turn it back on when necessary
8. Turn off Bluetooth, forget using Bluetooth headsets if you are trying to reserve battery
9. Turn off IrDA(infrared radio) and GPS
10. Minimize using all the bells & whistles: web browsing, camera, media player, photo viewing, etc.
I know these tips pretty much will turn your smartphone into a plain phone and for the most part, its asking you to refrain from using your phone in its full capacity. But, if you want your smartphone battery to last, these tips will hopefully keep your powered for as long as possible.
Tips to the manufacturers: just like sound profiles, it would be cool to have power-setting profiles; have one that incorporates all the tips plus more to maximize battery performance (kind of like many laptops do).
PS - A lot of smartphones can be charged via its USB/Data cable (ie. BlackBerry, Palm Treos, HTC), I always keep an extra USB cable (along with a jumpdrive, ethernet cable) in my business bag while traveling, this has helped me a few times.