I do remember hearing about a hormone that is typically a lot more present in the biological female body that makes things like the joints in the knees and hips a bit more flexible. That, coupled with the typical female tendency to be more fluid in their movements means that women just tend to look more fluid.
Like Reine said about her experiment in learning how to walk like a guy, she just moved a bit stiffer, but not enough for her to truly notice it. Just let yourself loose a little and you'll have the female walk down, I think. Another thing that might really help is getting at least a wedge type heel. Since walking in a heel requires placing your foot differently and that subtle difference in placing your foot can go all the way up to your hips and waist, it can naturally add a little feminine touch to your stride, especially since men typically don't wear heels.
