Why do I do the "saddle shuffle" when riding? I was browsing some time trial videos on YouTube and came across a video on saddle shuffling, and I've been noticing that I do the same saddle shuffle a lot when riding hard.  Did not really know it was a problem until I saw a video about it.  Is this common among riders?  My seat is correctly positioned too and it's perfectly level. I wonder how common this is...


                
            

To anyone who can't see the video: The problem is that a rider will, over time, shuffle forward on the saddle as they ride, every so often pushing themselves back so they're squarely on the saddle. 

Edit: About me:  I'm 5'6 about 145lb and ride a Specialized Allez Apex. I use clipless pedals and have received a BG Bike Fit. The bike size is a 52.  I would consider myself an aggresive rider.  

Update: So I thought I would update you with some tweaks I've done so far. I went to my LBS and purchased a shorter stem so I'm not leaning forward as much. I also measured my seat and myself (sit bones) and it showed that my seat was too narrow, though, I have been measured before and have always been told the seat was good (my other two bikes were the same). I was suggested to try the Romin or Avatar saddles. The look of the Romin seems like it will keep me from moving forward as much. I plan on testing the seats out in a trainer and around town. Thanks for all the suggestions.
It all depends on your saddle. As someone who has just been enlightened, some saddles are supposed to be pointed a bit nose up. Take the brooks flyer for instance, it is not the front that is supposed to be level. It is the back making the front point a bit upwards. I think of it less of a seat then a hammock. 

I have also had a saddle that was a left over from my fat days that provided extra back support. Because of it's design for bigger behinds, and me lacking one for the first time in a long time, I kept sliding forward which was hurting my taint (for lack of a better word).

Try angling your seat up a bit or maybe moving it forward a bit on the rails. Play around with the fit and see what kind of results you get. If all else fails you can go to your LBS and ask them to take a look at it for you.