I am assuming that plastic surgery is not an option.
1) Have good genetics. Not everyone can get 6-packs or 8-packs.
2) Do some intelligent strength-training. Lifting weights can definitely help.
3) For most people to have "tight abs," they have to have a relatively low body fat %. If it were healthy for you to do so (that is, not losing weight you should not be losing), you would need to lose more fat % and gain more lean muscle mass %.
4) Have patience. Healthy eating, intelligent exercising, and giving your stomach time to adjust can all combine to get rid of the "fatpack."
My mom says that when she was my age she was really fit. I've seen pictures and she's not kidding. But she never had a flat stomach, even though see did over 100 sit ups and did crew every day. I probably never will either. Something to keep in mind is that little bit of fat is very important when it comes to babies. It keeps them healthy and safe (or at least thinking that makes me feel better).
I do know that you can strengthen your lower abdominal shelf (which supports the baby) by doing leg lifts and cursive (lay on your back and raise your feet with straight legs about 4-6 inches off the ground then write your name with your feet in the air- the smaller the better). Doing exercises that strengthen that area might not get rid of the fat but it will make it stronger (adds definition and allows you to sit up taller and look thinner).
I think the best way to lose it is by rigorous regular cardio exercise. It is difficult to lose fat from one location while sparing it at other location. Running is particularly helpful.