Making yourself overly available trains those who would call upon you at unreasonable intervals to use your availability as a crutch.
"Plan ahead? Why? I can always call Bob!"
Unless you're in a position where you're on-call, don't check your work e-mail when you're 'off duty'.
4 a. Acquire FU money. b. Tell your boss's boss to go stuff themselves.
You would be surprised how few people lose their jobs by saying "no".
In either way though, there is a huge advantage to having choice - and being able to walk away from a job if push comes to shove. It affects the way you carry yourself, and it dramatically alters the dynamic between you and your employer.
The knowledge that, if things really got bad, if your boss turned out to be a giant asshole, you could walk away and be at another job in a week's time, is a tremendous psychological aid when it comes to pushing back and negotiating.