It sounds like your snake is getting over weight, without an actual picture it is real hard to say. First question..... How old is the snake?
Younger snakes that get fluffy can thin out real easy with adjustments to the feeding schedule and (beleive it or not) excercise. Take the snake out, handle it a little let it move and use up energy. If your feeding once a week go every 10 days until the snake looks like a loaf of round top bread.(like mentioned above)
Older snakes, like older people, take a liitle more careful dieting. Excercise, and adjust feeding out to every 2-3 weeks as long as there is no danger. (if you feed ever week slow the feeding down, 10 days then 14 days ect....). Until the adult hit a good weight/thickness. Breeding can also cause hips so remember to check the thickness of your snake, not just if your girls have hips.
Get a scale, weight your snake and document. Some do this weekly, I perfer at least once a month.
Once you accomplish getting the snake to a good weight, adjust the feeding schedule accordingly, if it is an adult Document the healthy weight in case he/she gets overweight later.
This is how I do it, but by no means is the only way to do it.