I just discovered this thread. I'm just on pg 7...maybe this has been mentioned already.
I like to keep large scraps of Blue Board (foam insulating board) and also cardboard handy. I use them to sit, kneel, or lay on while working on my vehicles & bikes. The blue board is waterproof, cushions, and keeps your butt warm, which is great if your working outside and/or on gravel or damp ground. It's also easy to wipe clean if you get it dirty.
I use the large cardboard scraps for the same purpose, but it's more disposable.
I'm also a big fan of rubber bands, large office binder clips, and magnets in the shop.