First, Get To Know Your Abs
Your abdominal area is made up of several muscles. These are all vital to functional movement, for a strong and stable core, and of course, for your womanly shape. The rectus abdominis gets the most attention, as this is the "six-pack" muscle. This muscle extends from your ribs to your hips, and is flanked on either side by the external obliques. The internal obliques connect the rectus abdominis and external obliques to your hips and pelvis. By focusing your efforts on training each muscle for the specific hourglass look, you can avoid sabotaging your goals.
Exercise to Avoid No.1: Situps
Unfortunately, the one exercise you must avoid to get an hourglass figure is the one most women force themselves to do, day in and day out. The criminal is the situp. Situps do very little to create a shapely midriff, and cause the exact opposite effect to what you want. Situps focus most heavily on the rectus abdominis. According to a 2009 paper from the University of New Mexico, frequent contraction of this muscle, as happens with situps, causes it to thicken and start to bulge. Unless you have very low body fat -- generally less than 12 percent for women, which is incredibly hard to achieve -- you will actually end up with a thicker stomach than you started with.
Exercise to Avoid No. 2: Any Weighted Oblique Work
Unfortunately, the myth of spot reduction has permeated much of the thinking about working out the waist area. Doing weighted work that focuses on the obliques may actually have the opposite effect to what you want; instead of removing love handles and creating a slim hourglass shape, you'll end up with a thicker set of obliques with a square shape. Chad Waterbury discusses this in his T-Nation article on "sexy female training" -- doing weighted sidebends and hyperextensions while aiming to get an hourglass figure just isn't going to happen.
The hourglass figure requires two things -- a flat belly and tight obliques. The best way to get these two features is to focus on a specific group of exercises that targets the rectus abdominis and external obliques without causing them to become bulky. Some of the best exercises for a tight, flat rectus abdominis muscle are myotatic crunches, the "cat vomit" and all of the plank variations. For shapely external obliques, focus on side planks, side leg raises and the bicycle maneuver.