Andrew is spot on. Speak to the providers regularly and threaten to take your custom elsewhere. Works a treat in most cases and where it doesn't you know you're doing the right thing moving to a new provider.
I've got TV from Sky, telephone and broadband form TalkTalk. I've tried shopping around and haggling and got the cheaper from the incumbents. The broadband is not great, but cheap and is sufficient for my limited usage. If I were using it for online video games, then it would be too slow. I'm not that sad, though.
The same thing applies to car (and any other) insurannce. Direct Line renewals quoted me £300 more than last year. I went to their web site and put in the same details and - hey presto - the price was exactly the same as last year's. One 2-minute phone call later, and all was the same.
Similarly, I have a Warranty Direct (Pinnacle Insurance) extended warranty on the motor. The quote was £649 online and £679 if I wanted instalments. I called, haggled, and got it for £499 spread over four equal quarerly payments with no interest.
You can tell I'm in procurement!