You might find the capstan equation interesting:
http://en.wikipedia.org/wiki/Capstan_equation
Basically having a few wraps around the moving part of your winch increases the maximum pulling force exponentially.
You might see a few photos of 5742's robot with a 6" diameter steel pulley on the front of it. There are three wraps of 1/8" plastic coated wire rope around there, and it does not slip.
http://www.chiefdelphi.com/media/photos/41356?
With two wraps it slipped a bit.
With one wrap is just spun uselessly.
You'll note that there is an eyebolt at the top of the 5742 lift to adjust tension in the cable. That also helps improve traction.
As far as linear slides are concerned, IGUS (the cool plastic bearing people) have some information on designing them at
http://www.igus.ca/wpck/6109/TechTalk_Overview Check out the "2:1 rule" and "How to Eliminate Binding"
Good luck,
Jason