Your BMI is in the normal range? Lucky you!
It's good that you catch the pounds you feel starting to pile on before it gets out of hand. Try not to get too worked up about it - you are of normal weight and you'd probably drop a dress size just taking a pilates class (for a few months) and still be in the normal weight range. Do you belong to a Y or a gym?
Resistance training or anything that strengthens your core is going to make you 'lean out' because your muscles will pull you up and make you stand taller (less 'slump'). Any weight loss would just be the low-cal icing on the cake and you wouldn't have to feel so driven to see a certain number on the scale!