How long have you been trying to improve your diet? If it was fairly recently then the cravings will go soon enough.
Just think of that fab body and clear skin you will have in the Summer. You will feel 100 times better and wonder why you ate that crap in the first place.
I'm not Gillian McKeith by any standard but do eat my five a day and exercise regularly, this means I can treat myself now and then. Don't cut all treats out of your diet or you will want them more.
I have never tried this myself but have heard that you should freeze individual squares of chocolate then when you have one pop it in your mouth and by the time it has melted the sugar has hit your system and the craving should have passed and you will have only had one incy bit of sweet stuff. I find having a glass of squash or juice helps as it takes away that sweetness craving. Or drink a pint of water, not only is it good for you but tends to bloat you a bit too so you won't feel like muching on crisps or whatever.
Keep at it though, you will feel so much better for having a healthier lifestyle but don't deprive yourself of all bad things or life wouldn't be worth living would it? ;-)