I'm 155lbs riding 48/16 stock pista and I can do seated skids, so as an average guy I hope I give some of you hope! As many have previously mentioned it's about building up the strength AND the technique. Just learn the timing of your individual bike: the cadence of your pedaling at various speeds, the position of the cranks at which point you can put the most force against the pedaling motion, etc etc. It's pretty intuitive after you ride your bike for a while and there's a natural progression from nuts-do-the-stem skidding and seated skids. Just slowly bring your weight back towards the seat and eventually you will get it. Good luck!