If you're sliding forward, it could be the tilt of the saddle. Try raising the nose just a hair - if you don't have a micro-adjustable seatpost, you can shim the tilt with a little bit of metal. I cut a rectangle from the top of a steel can and folded it over once; aluminum tends to compress too much to be useful as a shim.
Alternately, it could be that the saddle is too pear-shaped for you (gradual transition from the back to the nose of the saddle). Pear-shaped saddles will force me forward onto the nose. It doesn't matter whether the back is wide enough for my sitbones, because I can't sit on the back and pedal.
IIRC, Selle Italia makes several different women's gel saddles, but you can look up the specs on yours to see how wide it was. If you actually make a print of your sit-bones, you can see that there's a fairly large area that can take your weight; normally you'll measure center-to-center, but you may want a saddle wide enough to support all the way to the edges, which is usually at least 20 mm wider than your center-to-center measurement and may be more than that. Remember that saddles are measured at the widest point, but often slope toward the edges - so Biciclista is right, you want the brunt of your weight solidly supported.
Speed comes from what you put behind you. - Judi Ketteler