Tried these today for the first time.
http://www.ronniesunshines.com/armour-flex-knee-pads-black-p-963.html
admittedly i only had them on for 2 hrs but they never moved were very comfy and protected my knees really well.
They have an outer of 3mm plastic, with hinge pin at the sides of the knees. The lower half has 10mm high density foam insert, more than enough to pad the knees when going to ground. Additionally the lower half has a high friction rubber section that prevents slipping nicley. The hinges work like a charm i barely knew they were there actually! The elasticated strap