EVEN IF YOU DON'T WANT TO SWITCH, NEGOTIATE.
If your phone is out of contract, or coming up for renewal, then make some time to start haggling on your contract with your current provider - you'll be amazed at the deals you can get.
If you are going to negotiate, it can be helpful to arm yourself with some facts first. Make sure you know your usage and what you need. You can use comparison sites to look around to see what deals are on offer, so you can use these as example benchmarks.
If you are happy with your handset then it's easier to haggle the prices down as new contracts often include the cost of the phone within them (or if you want an upgrade, don't necessarily go for the latest model). You could also consider looking at Sim only deals.
Don't be afraid to negotiate - you don't have to give up your contract. Push on the cost, term and inclusions. If you aren't happy or want to see what else is available from other providers then simply tell them you will think about it and let them know.