The term would be "the shuffle". Check out the video in the link below. These riders are sliding forward on the saddle for several reasons.

The time trial shuffle:

-UCI does not allow them to slide the saddle far enough forward on the bike. The rules are that the tip of the saddle must be at least 5cm behind plumb to bottom bracket. The riders basically WANT the saddle to be further forward because their bars are extremely low creating more distance from the saddle.
-They are in such a low position, that all of their weight is on their arms/upper body. This causes the rider to naturally shift forward because there is very little weight actually on the saddle.

For you, the issue can also be the angle of the saddle. If it is pointed even a bit too far down, it will cause your weight to shift more into your hands and you WILL slide with the seat. People tilt saddles down anywhere from 1degree to 3 degrees to relieve perineal pressure and it can help, but you have to compensate by keeping your lower back tight to prevent your sacrum from tucking under. This does require more lower back strength. If you get pressure with a flat saddle and are tilting it down to relieve it, you may want to experiment with more saddle choices. The perfect saddle will give you little to no perineal pressure by with 0--1 deg of tilt. The Adamos are famous for helping with this. Maybe visit a good shop and they will have test saddles to try and possibly purchase.

Yes, I am a certified fitter. More info on that is below.
http://cnj.craigslist.org/bik/5767293727.html