Keeping power on your device is always a good thing and when your using a smartphone your screen will eat up your batter before the end of the day.
There are apps out there that will help you increase your battery life by managing your services and wireless devices.
SetCPU for Root Users
Watchdog Task Manager
Though these are not perfect and don't always improve your battery life though they do help.
The sure way of increasing your battery life is to actually increase your battery. If your wanting the same slim look that you had before you can replace the battery though out the day with another battery. You can find a replacement battery for Galaxy s3 here:
Replacement for a galaxy s4 battery here:
The prices are good and you just need to pop off the back and then replace it. Though not all smartphones have replaceable batteries like that of the galaxy line.
If your looking for a battery that will charge your phone 4x over or charge your Tablet then you will want to check out
This brand is what I use for the extended batteries for the galaxy s3 i have and also for the external battery that my wife uses on her HTC One x.
They have very good customer service and stand behind their battery.
Whats your method of increasing your battery supply?
-Local Tech Repair Admin