Why do people worry about turning wi-fi and bluetooth on and off? This isn't Android, and even if it were, that OS is much more efficient now than it was in its infancy.
Having wi-fi on does not affect your battery life as dramatically as you may think. Probably not even as much as you spend actually worrying about switching it on and off.
Same goes for GPS.
Just leave it on. If your phone needs to use it, it will, and the icon will display on your task bar. If not, it won't be doing anything. Not even draining your battery.
3G is
way more battery-intensive than wi-fi, especially if your signal is weak.
Relax, guys. The days of task-killers, GPS toggles, and wi-fi toggles are behind us. Don't worry about it, and you'll realize everything is fine
Even if you're in a wi-fi zone that is giving you a weak signal and you'd rather use 3G, just go into your settings and "forget" that network. No need to disable wi-fi completely.