Renegotiate your sim only contract - Virgin
Some people believe you cannot negotiate your phone package but this is untrue.
Even if your on a sim only 30 day rolling contract you could still get a deal.
My existing package was 800 mins 3000 texts and 1GB of internet for £10.44 per month.
I called up Virgin Mobile and was offered 800 mins 2500 texts and 2.5GB of internet for £8.99 a month. I also use a student nus card for an extra 15% off.
They mentioned the deals available vary per customer but as I only have 2 Sim only contracts with them I wouldn't have thought I'd be entitled to the best deals. Make sure you tell them if you have virgin media.
However i tried to renegotiate another contract with Tesco Mobile and they point blank refused so it obviously depends on the network and your relationship with them.
EDIT: Make sure you speak to the cancellation/retention team
Please post any success stories you have had.