For anybody who's not aware, if you're on a monthly contract, and haven't taken an upgrade, you're throwing money away.
Most contracts include a figure which is basically financing your 'free upgrade' every year or so. If you don't want an upgrade, ask to be put onto a sim only contract, this means you are not tied in, and should reduce your line rental by £10 per month or thereabout. The exception to this could be if you haggle hard every year when your contract comes up.
My other tip, having worked in the industry, is that when your contract is up for renewal, request your pac code, stating that you wish to leave to go to another network. It helps if you've looked up a competitors deal which is cheaper.
Regardless of whether or not you would wish to change network, requesting your pac code indicates to your current provider that you are seriously considering leaving, you then get a telephone call from their retention teams, who are empowered to give you much better terms than the renewal teams. Sadly very few corporations reward 'loyalty' nowadays, unless your force them to!