Some of my buddies have expressed astonishment that I get approximately thirty-six hours of standby with my iPhone, so I'm sharing my settings. Note: this is my second 3G iPhone--I returned the first one because I was getting only five hours of standby time.
I don't know if you'll get this kind of standby time (or more!), but this is what works for me. Of course, people with lesser phones are having a good chuckle because they get three to five days standby time, but their phones aren't cool. And it's better to have a cool phone that you have to charge everyday than a sucky phone that you can charge once a week.