I used the helpful Moneysavingexpert.com Cheap Energy Club to keep track of whether I was getting the best deal for my power. If you know roughly how much power you use, or even if it is a guess, they will let you know when you are paying over the odds for it. Whether you swap provider is up to you.
When I moved home I changed supplier to Octopus, who were cheapest at that time. Well, the costs went up last year, for everyone I think, but I am sticking with Octopus. I have never had such excellent customer service. Polite, cheerful, individual, and they really do listen to (or read) what it is you are telling them. They prompt you to give meter readings every month and send you an energy usage statement in response so you can keep track of your use and spending. They even have a little 'wheel of fortune' to play when you've sent a meter reading, so you might even win a few hundred quid! Their electricity is now entirely green- generated and the tariff I'm on is STILL an excellent deal.
Switching is easy, no matter which supplier you choose, and I used to do it often. Don't be shy, you don't lose any power while the switch is going through and you could save hundreds!