Hi,
I have found that by adding kickboxing to my regular cardio routine has done WONDERS for slimming down the entire waist/hips/buttocks area on the whole. I do a combo of step/hi-low and kickboxing, but I do at least 2 kickboxing cardios/week. Until I added the kickboxing, my waistline measurement didn't seem to want to budge at all! Additionally, kickboxing strengthens your abs, which has in turn helped me with other exercises like planks.
Donna