I would agree with Art. Conditioning and building up your legs is better than "crutches." A hiking staff can help a great deal in increasing stability - so can just plain old experience. Good solid footgear plays a role as well.

I use kneepads myself, since I spend a lot of time on my knees digging up bones and such (as well as begging for forgiveness). The most practical for outdoors use are the light foam types one can get at the local home center. You can also find pants that incorporate knee pads.

Cavers also spend a lot of time crawling, often in mud. The heavier construction type pads work best in that situation.