sliding forward on the saddle is the one thing that bugs the hell out of me, i've been doing it since i started riding last year, but since i discovered the saddle was a bit far back it's not nearly as bad as it used to be. i have wide sit bones, so i find that to make my saddle comfortable i need to sit further back on it, so i slid the saddle forward so i could sit further back on it still have knee over my pedal spindle. so far, so good...

i also switched to a shorter (10mm) stem, because i thought i was sliding forward because i was reaching to far, but after i changed it i was still sliding forward so that wasn't the reason for it...