For a chair, I use this guy:
http://smile.amazon.com/Vertagear-Racing-S-Line-SL2000-Ergonomic/dp/B012PTL1SI?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
Mine's an awesome pink color that they don't seem to sell anymore
For rudder pedals or the like, if you don't want them to slip on a hardwood floor, buy one of these, cut it into 4 pieces, and put one piece under each corner. Works really well and saves some building! I also do this under my PFC Yoke to keep it from sliding around on the desk, and it works amazing!
http://smile.amazon.com/Heininger-1065-CommuteMate-DashGrip/dp/B003A8D6YG?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
-stefan