I have tried this with the iPhone 4, but will work on any other smartphones as well.
Do you really need 3g connected all the time? NO you don’t. So, go ahead and de-activate it in the Settings > Mobile Data; Activate 3G.
In my experience, you will double the battery!!! If at some point you need to upload or download a big file, just activate it again, it only takes 5 seconds.
Deactivate Push E-mail
In the iPhone, you can choose to have email checked every 15m, 30m, … I have selected 15m and it is more than enough! You can always open the email client and refresh!
I followed these and other recommendations found in Battery Life Pro App.