I know this isn't a spell, but sluggish metabolism can be linked to an underactive thyroid gland, you might want to check it with your doctor before trying spells. If it's all healthy, and you're living an active lifestyle with a good diet, then look through the health spells listed on the site, there are many! :D
As Lady Zeb said, a healthy life style is key to maintaining an active, strong metabolism. Excessive intake of junk food, spending extended time relaxing with no counter (exercising) can be problematic. Alternatives would include foods that naturally have a low calory content to match the ones with high ones, and then choosing some options that make you walk a little more or reach a little higher as basic, simple ways to incorporate healthier habits into a lifestyle.
I have a ritual of healing that is for general health. I'll mail it to you when I get to the house this afternoon. If you haven't looked into the basics yet, there are some links and articles on my profile. You may want to check those out first, as they are very important to the ritual.
I'd recommend looking into incorporating herbs that boost metabolism and promote good digestive health. Also, greek yogurt is AWESOME for regulating your digestive system, which in turn will speed up your metabolism. Like mentioned above, you want to make sure that you're living a healthy, active lifestyle, and check in with your doctor to make sure this isn't a sign of some larger health concern. Intuitive eating, high protein snacks every 2-3 hours, regulating your body clock...The key to a good metabolism is maintaining a healthy alignment within your entire body system.