It's not often I feel you can win against the phone companies.  When our mobile phone bills arrived this month, we saw that my husband was now paying substantially less a month in line rental than I was.  He'd been rung by our provider and offered a new 18 month package, a new phone and a discount.
So I rang the phone company to ask for the same deal.  The guy I spoke to said that deal had ended, but he could offer me something similar for £5 more than my husband is paying. It was still less than I was paying, but I decided I had nothing to lose.
I mentioned that prior to my current contract I had been with another phone company with whom I had been happy. I had only moved because of the joint deal that my husband and I had been offered.  I would need to take this into consideration before agreeing to anything further with them.
I was immediately passed through to the "customer loyalty" department.
I was offered the same deal as my husband, and I negotiated a little more off it - I now pay £15 less than the package cost and £1.20 a month less than my husband!  I'm also getting unlimited texts, whereas he has a limit (albeit a ridiculously high one).
So it's definitely worth taking them on if you feel you are being screwed over, and for the price of a new contract (which you'd need to take out with someone anyway) you can get a good deal if you are willing to be polite but firm.