Create a powerful goal. Write it down and read it at least twice a day. Here's an example: "By April 10th I will lose 15 pounds in a healthy way by eating cleanly and using efficient exercise."
Focus like a laser. The more you focus on your goal, the faster you'll achieve it. Focus is a key to fast weight loss success.
Visualize success. Spend a few minutes each day visualizing yourself after having lost 15 pounds. Think about how great you'll look and feel after losing those ugly and dangerous pounds of fat. Create a "mental movie" of your success and play it in your mind whenever you need a motivation boost.
Build momentum with early "wins". Set small mini-goals for each day and week. Write them all down and check them off as you go through your daily routine. Get a calendar and mark a line through each day that you reach all your mini-goals. This is a great way to build the kind of motivation you'll need to lose those 15 pounds quickly!
Boost energy naturally. You'll need lots of energy to lose large amounts of stubborn body fat. Boost your energy naturally by getting plenty of sleep, avoiding simple sugars and refined/processed carbohydrates, drinking at least 60 ounces of water per day, eating protein with each meal and snack, doing some refreshing morning exercise each day, and avoiding alcohol as much as possible.
Eat a paleo-style diet. Get almost all of your calories from lean proteins, natural carbs (veggies, fruits, beans and legumes), and healthy fats (olive oil, coconut oil, avocado, nuts, seeds, etc.). Avoid all processed foods as much as possible. Avoid grain products and other starches for the time being, at least 2-3 weeks. Then cycle them back into your diet, choosing only the best starches (whole grains, potatoes, oatmeal, etc.).
Do intense workouts often. Several times a week do a short, intense workout based around bursts of activity (aka "intervals"). These work incredibly well for burning fat and ramping up the metabolism in a way that allows you to continue losing pounds even while you're asleep. Do both strength/weight training and HIIT-style cardio. At least once a week do a "fun cardio" activity, such as playing a sport or taking an exercise class at the gym.
Take whey and omega 3 fats. The only supplements you should really consider taking for faster and healthier weight loss are whey protein powder and omega 3 fats. Whey is great for post-workout recovery and for making healthy protein shakes. Omega 3 fats are an essential nutrient you need for good health, high energy, and a fast metabolism.
That's in, a quick overview of the best way to lose 15 pounds fast. Incorporate them into your fitness plan and you'll have much better success. Goals, focus, momentum, high energy, clean diet, intense workouts, and proven supplements are your keys to a leaner, fitter, healthier, sexier-looking body. Good luck!