I'm 29. I've been eeking and creeking since I was 16. My friends used to tease me and say I acted like an old man. Overall I'm in pretty good shape, but I've had problems with my back, wrists, and a little with my knees.
As far as the knees are concerned: CRW, you've got to be mechanically minded being in the trades and all. The knee is poorly designed to be used as a peer or footing to support the weight of your body, not to mention any excess weight. When you say:
put any weight on them kneeling, they start aching and they get messed up for a few days or longer
I feel your pain. I've been told from many fitness training type people that when ever the knee is bent more than 90 degrees, it puts tremendous stress on the joint, especially under load (weight training or working hard). Just going into a squat for putting outlets in can stress them, moreso if you carry a heavy load (tools or belly). I used to get razzed for sitting at each outlet, but now I'm the boss, so it doesn't matter anymore.
I avoid straining my knees as much as possible. I bought a creeper for one job ($30) but I know most attics and crawls aren't a smooth hard surface. The one I got is a seat on wheels and you can put tools and parts on a small platform right below and slightly in front of you.
Also I've found that by exercising, my aches aren't as bad. For knees try something low impact like cycling. Set the seat so on the down stroke your knee is still barely bent so you don't hyper extend (lock). You need to increase blood flow to the effected area to promote healing.
I like the pads idea. I should probably pick some up.
My only other suggestion would be to get a helper and do more pointing.
