First of all, and just like any other physical fitness program - excess fat around the middle is a problem. So if you're still eating junk and slighting yourself valuable nutrition, you're likely not going to see the results you're hoping for.
That said, let's move on. If you're noticing changes in your shape, but you think your waist could be smaller, consider your abdominal workout. The standard crunch and the regular situp are great exercises and you should keep doing them- but they DON'T maximize your abdominal workout, nor do they adequately work the transverse abdominals. You will need to alter your routine to include exercises that maximize your internal corset.
Pelvic Tilts - Lay on your back on the floor or a mat. Lift your knees to about 90 degrees while keeping your feet flat on the floor. Raise only your pelvis, keeping your lower back on the ground. Hold it for three or four seconds and lower it again. Repeat for two or three sets - depending on your level of fitness.
The Crunch-less Crunch (aka Stomach Vacuum)- This one is great because you can do it any time and anywhere ... basically you're going to pull your belly button toward your spine. Sit up straight and concentrate on drawing in the part of your stomach that is just below the belly button - continue to draw yourself in until you feel your belly button being pulled toward your spine. Relax and repeat as many times as you can. You can do this exercise standing, but because it tends to take practice - you should start in a sitting or lying position (if you do this lying down, bend your knees and keep your feet on the floor).
Scissor Kicks- While lying on the floor place your hands under your tush. Keep your back pressed against the floor. Slowly raise one leg to a height of about one foot from the floor, then slowly lower it back. As your lower one leg, raise the other. Like scissors cutting the air. Repeat this motion for an entire set. Control is important, don't allow momentum to increase rapidly. Your upper body should remain on the floor through the complete exercise.
Oblique Bridge and Standard Plank- Lie on your side while holding your upper body off the ground with your elbow. Keeping your legs together, raise your midsection off the floor ... your torso should be at about a 45 degree angle from the floor, your feet and elbow being the only points touching the floor. Keep your body as straight as possible - Hold for a count of ten (if you can) lower yourself and repeat until you've done three on each side. After you've done this, roll onto your stomach and hold your upper body off the ground with both elbows. Position your feet as though you were about to do a pushup (toes on the floor) and lift your torso off the ground, again to a straight 45 degree angle. Hold for a count of ten and repeat three times if you can. (This little series will rock your world!)
In addition to these basic exercises, there are some additional tips for whittling your waist. (This is great for women who have had children and have stretched or separated abdominals), get a sheet or long blanket. In my experience, the standard beach towel is not big enough. You need a long, wide strip of fabric. Hold the fabric behind your body as if you were going to use it as a belt, then lie on the floor. Cross the fabric in front of you as if you were wrapping yourself in it. You will be holding the left side of the fabric in your right hand and the right side of the fabric in your left hand. Pull the fabric tight around your middle as if you were tightening a corset and slowly lift your head off the floor to engage your abdominals. Do not bend your legs. This exercise is hard to visualize, but it works. Basically, you're pulling your rectus abdominals back together and tightening them at the same time.
One more trick is to actually wear a corset or a body shaper as often as possible. It seems nutty, but it actually will help your body "remember" a naturally small waist. Because we have no ribs around the belly, and we eat so much we stretch it all out, wearing a compression garment can help push all that back where it belongs and can even put some pressure on your stomach that will help you feel full faster during meals.
Treat your skin well, take your supplements, and follow a regimented abdominal routine ... also, you might consider trying to tan this area, either naturally in the sun (my favorite method), tanning bed, or with a sunless tanner. A sun kissed skin tone will make your waist appear smaller even before you've done the work to get it there!
Remember, a small waist isn't just for the ladies - these exercises are very beneficial for men too ... they will help improve posture and ease back pain in addition to creating that torso we all want!