The way I learned how to crab is just slowy get the movement done.
Like start with your pinkie and hit the fader open then move to ring .. etc...
when you get to the index send the fader to the middle.
Just go through the movements slowy and make sure your cuttin off the sound, speed will come later. And I mean slowy go through it. Do it till your hand cramps. Also when your not scratchin pratice the movements with your fingers touching your thumb. Try to get the even slide movement and pretend your thumbs on the fader. This thing here is a good way to practice and build strengh
http://www.zzounds.com/item--PTWGMM
also you should cop
Contact/Control Cleaner & Lubricant from Radio Shack and give a squirt into all your faders
It makes your fader move and stops em from bleeding sound
And one last thing go buy new slipmats and some wax paper. Cut a slipmat out of wax paper than put it under your new slipmat.