The headset (vertical) should be higher and the stem (horizontal) should be shorter on a women's bike than on one for a male of the same height, as both are determined by arm length. Arm length varies substantially between women and men, so the optimal bar placement varies a great deal as well. While the lower and farther that you are able to comfortably reach, the better in terms of aerodynamics, don't get stuck with handlebars in the wrong spot just because you're a girl.

While the headset's vertical height can be adjusted, the length of your stem is fixed, so buy one of the correct size the first time. Or, better yet, use an adjustable stem until you find the right length. The height and length settings are related, of course, so they can compensate for one another to an extent.

Handlebar position is generally set by the nose being directly above handlebars when down on the hooks. A second rule of thumb is when riding in you normal position, your front hub should appear hidden by or a bit behind your handlebars. Alternately, sit on the bike in your normal riding position while someone holds it steady. Without changing position, remove one hand from the bars and let it relax and dangle freely. Without stretching, rotate your arm in a large arc. As it comes back to the bar, if it comes ahead or behind your other hand, your handlebars need to be moved.