It's a hard thing to find balance. Once you have become someone the schedulers know they can get to work OT, its hard to get them to stop. But a few times of saying NO and it will get easier to do. Not only that, your confidence in saying NO will grow.
Saying no is not a major thing, but we as nurturing people have a difficult time with that little word.
You need your family, pets, friends, hobbies, homelife, etc to keep you grounded and focused on what's really important in life.
A trick I use: I only give my cell phone number to work, never my home. Cell phone has the caller id and voice mail on it. I can see who is calling, choose not to answer it, and ignore any voice mail I choose. Screening your calls is a beautiful thing.