Ahh in that case there is two options. You can upgrade or buy a new iPhone. Or you could try taking the phone to Apple if the problem persists and have them check the battery. If its running at under 50% capacity I believe they will consider replacing the battery for a fee, don't think its laods though, you may even be able to get a cheaper price if you mention that iOS5 is the core problem.
Sorry i couldn't be more help.