Wi-Fi is one of the biggest power hogs on your phone so it's best to turn it off when not in use. But who can remember to do that?
One of the simplest Wi-Fi power-saving apps out there is Wi-Fi Power Saver, which turns off Wi-Fi when no network is detected. You don't even have to configure it unless you want to.
For more options, the GreenPower Premium app uses location to automatically turn on Wi-Fi when in a designated hotspot, and then turn it off when not in range. It also manages Bluetooth.Wi-Fi Power Saver (free). Green Power Premium (99 cents).