There are many carpentry tasks that require getting down on one or both knees, including installing deck boards, railings, and trim. To make life easier for my aching knees, I purchased some foam swim kick boards (the boards that are used to teach kids to swim) at my local Dollar Store. At a buck each, it turned out to be money well-spent. The boards are perfect for kneeling or sitting on and are lightweight, and they don't take up much room in my truck. But the best part is that they protect my knees - no more kneeling on cold concrete or hard flooring. If you can't find any locally, you could try a discount pool store, or you can cut some out of old rigid foam insulation if you have any lying around.