Apple has admitted to battery-sucking bugs in its recently released iOS 5, has promised a fix, which was released last Thursday (iOS 5.0.1).
But after upgrading, very likely, you still have the same issue. On some forums, some users even reported the update simply made the situation worse.
For the time being, you can not do much if your iPhone 4S unluckily has this issue, because Apple did not release the cause of the problem, as usual. Or perhaps, they haven’t found the real causes yet.