I did lose weight once on a solely magical endeavour but the loss as minimal and easily gained back. Magic tends to work better in this area when it is supporting practical steps which usually (and very unfortunately for those of us who like our food) involves eating less... :(
There are a couple of magical suggestions I can make. You could try dedicating a large pillar candle to the purpose, customising it with items associated with yourself (I used buttons to represent the buttons on my skirt that wouldn't do up!). Burn the candle a couple of times a week for a 15 minute period while visualising the reduction in the candle representing the reduction in your body fat. Pour away the liquified fat at the end of each session of working with the candle and the idea is that when the candle finally melts away it will coincide with your excess fat leaving your body.
You could also try binding your own appetite. Try taking pictures of all the different foods you like (you can always print them off the internet) and once you've got a pile of pictures representing all your naughty food tastes roll them into a cylinder and tie securely in the middle. Breathe into one end of the tube and as you do visualise all your desire for unhealthy food passing from you and into the tube. Seal the tube of pictures up with candle wax and place somewhere safe.
It isn't exactly magic but something else that I am advised works well is having the suggestion of gastric band implanted in your mind via hypnosis. I haven't tried this personally but I've spoken to people who have and they have confirmed that the appetite is reduced so you feel full after eating only a small amount of food. Might be worth a go, hypnosis is likely to be cheaper than a course of diet pills and won't have the side effects that a lot of chemical solutions to weight loss come with.
While I would agree it is important to take your health seriously I do have to add in close that being overweight need not be a barrier to body image or happiness. If you lose weight do it for you and not to conform to a society stereotype. Beauty isn't all about the waistline and sometimes as women it is prudent to remind ourselves of that fact.