Smart Phone is like a car, when not in use, shutdown non essential things. Most important things for me are call, SMS, and MMS. Thus, network radio signal needs to be on. 3G Data, Background data, Auto Sync, GPS, Wi-Fi, Bluetooth, etc. can remain off when not in use. Especially, at night why keep 3G and Background data on?
If wi-fi or GPS or 3G is on and any of these signals are weak then the phone drains too much battery. We have to ensure to turn off wi-fi or GPS or 3G if any of those signals are weak.
Many times I have done cardinal sin of stopping applications and app services too just because phone was heating too much.
Of course, Playing too much music, videos, games and browsing will certainly eat up the battery.
The main idea is to have enough phone battery power available for essential tasks when we need to perform them.