After the update to iOS 5.1.1, some users complained that the battery life of their terminals was not better than in the case of iOS 5.0.x or 5.1. Universal methods for solving this "problem" do not exist, but for those who did the OTA Update to iOS 5.1.1 there could be a very simple method to solve everything, but of course you are not sure that it will work. Before applying this method, it is advisable to make a backup of your device through iTunes or iCloud because you will lose all saved Wi-Fi passwords.
If you have made a backup or you do not care to remember those passwords, then go in Settings>General>Reset and choose the option Reset Network Settings. The phone will reset and after reloading the operating system you should notice an improvement in the battery life of your device. If it still doesn't work, then you can do a restore to iOS 5.1.1 and you can avoid reinstalling the device backup after the restore, and that way you will lose all the data, but you could have a better autonomy.
I'm coming back and telling you that the method only worked for those who did the OTA Update to iOS 5.1.1, so if you try it on a terminal where you updated from iTunes or restored from iTunes, then there is a good chance that the result will not be the expected one.
