I think abs are generally one of the hardest areas to target, especially for woman who naturally store more body fat than men and tend to hold it around the lower stomach. Everyone's perception of abs is very different and the look people want from their stomach.
Most women you ask want a flat stomach, not a "6 pack".
I think a lot of women think that crunches alone will achieve this without realising the structure of the stomach muscles and how to engage other muscles such as the obliques.
The internal and external obliques are the muscles that contribute to the toned side of the stomach most woman desire, working these along with lower abs achieves a more feminine and toned appearance.
An example of that could be Britney, she had an amazing stomach back in the day, you can see how toned her obliques are without looking to masculine and having a defined 6 pack.
As well as working the core, to tighten the whole mid section as a whole, I've found that working my obliques have helped change my body shape in addition creating a smaller/tighter waist.
My stomach was one of my worst areas and feel it's an area I've really worked on the past 2 years and it's starting to show!
A Few Exercises That I Do
Side Plank With a Twist
This takes a lot of core strength and stability, I do 8 each side, being really careful not to bend the back and keep the core tight.
Using The Stability Ball/Weighted Ball
I like to do leg raises with a stability ball to help work the core and use my lower abs to control the ball.
Also, using a weighted ball for sit-ups and reaching high with ball and back down over head I feel works the full upper and lower abs.
Plank with Bosu Ball
The plank can be really versatile - I like to use a Bosu ball to help engage the core and work on stability, although you can go without and just use a mat or the floor if you don't have access to one.
Crunches / Side Crunches
Standard crunches are great for upper and lower abs. I like to do side crunches to really work the obliques too
Weighted Side Bends
I tend to do side bends in every session between sets as my rest. It's important to make sure you keep the back straight and really think and concentrate on the obliques.
For lower abs I like to do leg raises to failure, ensuring the movement is slow and controlled - making sure the feet and ankles don't rest on the floor.
...and those are a few exercises to consider, if you do these a few times a week and really push yourself you'll be on the way to great set of Abs! :)