Turning 3G off
Although I'm not using any push service nor other background applications, the first thing I tried to turn off was 3G. It surely depends on hoy good the network coverage where you live. In my case, at office we've got a pretty bad coverage and 3G seems to improve it very much. Deactivating that option made thing go worse and battery drain was not reduced. This weekend, at home, I turned it off again and here, with a good and stable coverage, things got a lot better.
Turning WIFI off
I noticed that, even if I only connect to a wireless network at home, the iPhone would probably be scanning the networks all day long. Deactivating WIFI when not using it greatly improved battery life. I consider this to be the setting which impacted me most.