Always disable autoplay and use theatre mode, even when browsing incognito.
Incognito mode is great when you want to browse with a clean slate. However, this also means that all websites you visit will have forgotten your preferences.
This extension simply resets YouTube to widescreen (or "Theatre" or "Cinema") mode, and unticks autoplay. It also disables autoplay in Vimeo.
NOTE: in order to work in Incognito mode, remember to check "Allow in incognito" in the extensions tab.