When it comes to GPS, have it off at all times unless you need to use it. ANd as for app killers, that's where it's abit of a grey area.
From what I've heard, killing some apps doesn't help and actually causes you to use more CPU which means more battery drain.
For example: Say you killed a weather app and about an hour later, it starts again and you kill it. And so on for the next few hours. It takes more energy having it start and stop all those times then it does just letting it run without being killed.