It's not just the saddle itself, it's the tilt, the length of your stem.... And you'll sit differently on every different saddle, so I don't know that there's an answer to your question really.

My guess is, if your saddle is a size and shape that allows you to sit on your sit bones, but you're not doing it, the saddle probably isn't the culprit. Try tilting the nose up just a bit. If you don't have a micro-adjustable seatpost, you can shim the tilt with a little piece of metal cut from a can (steel can is best because it's thicker and won't compress as much, but you can use aluminum, folded in half or fourths). Especially if you're constantly sliding forward on a T-shaped saddle, a tilt adjustment might help.

Off topic - but Aggie, maybe just some shallower drops would be useful. The stock bars on our bike are rather deep. Obviously it wouldn't put you in as low an aero tuck as your stock bars, but if you can be comfortable, it'll give you a better position for pacelining and descending. Earlier this year I put on a set of Ritchey Biomax Pro bars (thanks, Liza!) - a bit wider but also much shallower and ergo drops - and I'm really loving them.