You need to focus more on your nutrition. Two great resources are Tosca Reno's Eat Clean Diet series (really strict but will get you the results you want) or The ABS Diet (not really a 'diet' but an eating plan you can adopt for a lifetime that focuses on foods that do not bloat, but promote fat burning).
Unfortunately, you cannot spot reduce or spot train. Great nutrition will help burn fat all over the body but women tend to hold on to the fat in the abdomen and thighs. It'll likely be the last to go.
Crunches may not be enough. Start out with the Tupler Technique (as mentioned below - she has a book called "Lose the Mummy Tummy", get it from the library as it is sort of a waste of money because it repeats itself and doesn't really give you much more than an absolute beginner workout) but then you'll need to progress to other exercises that are more challenging. A lot of the techniques within target the transverse abdominus muscle. It wraps around the midsection like a corset - so imagine what a corset does when pulled tight around a woman's waist. It gives the appearance of a narrower waist. Functionally, however, it provides a lot of core strength and stability for every day movements. Planks, side planks are all good things to do.
And, crunches alone won't cut it. They will tighten and strengthen the midsection but if you have a nice layer of fat insulating the 6-pack you won't ever see it! You need to continue to do the cardio that burns fat. Mix it up on the treadmill and try more interval training - play with the speed, time intervals, and incline (do NOT hold onto the treadmill when you work out - lower your center of gravity and pump your arms).
