Why haven't you learnt how to buy a phone?
Mobile networks are ready to crap all over you, but only if you let them. I am amazed and frustrated at the sheer number of people who don't think twice before throwing away their hard-earned.
it's simple maths
Really, it's not hard. Cost of the contract multiplied by the length of the contract, plus any extra charge for the device. Sometimes it's worth paying more monthly for a cheaper/free phone, but this is the exception - £5 more a month is over £100 more over the life of the contract...
don't overbuy (or get over-sold)
Seriously, how many minutes and texts do you really need? I cringe when somebody tells me they got [x hundred] more minutes because it was only £10 more (not because they use or need it). That's £180/£240 depending on contract length, it's simply bad value if you don't use it. Before any renewal I check all statements, know exactly what I use and need.
If you start a 'renewal' conversation with your mobile network with any other words you are doing it wrong. Look around, know exactly what others can give you and find the very best deal. They will beat it if they want your custom, they won't know and/or check the coverage in your area so you can be generous with the truth.
Sorry for the rant, it just gets to me. I renewed my contract again today and people always seem amazed at the deals I can get. It's not amazing, it's just common sense. I am paying £8/month for 600 mins / 600 txt / Unlimited net on a 12 month sim only. Not getting a phone from the network means I can change when I want, I get updates sooner and typically save ~£140-200 a year compared to a phone included with a contract.