I agree with the idea of writing a list as to why you want to lose weight.
I sat down last week and wrote a list, I put it on my computer so it is entirely private. I literally listed everything reason in the world for losing weight, from big important reasons like improving my health right down to really silly and tiny reasons like not having to buy tights in size XXL!
There were so many reasons, every time I look at it I realise that sacrificing a fatty diet is going to give me so much in return. Its a really good tool and I recommend it. Then whenever you feel like going off the rails you can say 'is this biscuit/cheese/cake etc.. worth more to me than all those reasons I identified for losing weight?'