In my experience, breaking it out of muscle memory does the trick. I've done this with sites like Reddit by blocking the domains, blocking from router, etc.

With that said, I've found the hardest ones to block are the ones I like for research, staying informed, etc. Namely, Reddit and HN. I can't block them because I find out too much useful things from them, both passively and actively. Yet, allowing access to Reddit eventually leads me into the worthless, junk food information feed that is /r/all.

So.. there's my partial answer.