The best way to lose your butt is to change your diet. Eat more meals (3-5 per day) but each less at each meal. Don't eat after 6 p.m. Have your snacks mid-day instead of in the evening or after dinner. Try low carb during your last meal of the day.
As for exercises, cardio is best for the thighs and butt. The stationery bike is a good one. Rollerblading can help because you are getting a workout in those muscles, but I would also do some stationery bike or treadmill exercises. Keep the resistance low though as you do these exercises. Otherwise, you'll be building muscle strength instead of losing fat. Try to get about 20 minutes of good cardio exercise. If you're not used to that, start with 10 or 15 minutes and work your way up to it by adding a couple of minutes per week.
Don't just work on your butt, though. You need to work out your whole body. By doing a full body work out you can actually lose fat all around. The butt is one of the first places fat will grow and one of the first where you will lose it so exercising the whole body will cause fat reduction. But the main thing is diet. Don't go for the fad diets. Just modify your current diet so that you are eating the right foods at the right time.
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