I list a director, a production manager and 2 Stage managers I worked under. Although, I do sometimes change it up, change people out depending on the gig. I've tried to give it a bit of balence as well so not ALL of my references are coming from the same theatre. On my resume, I list the people and their position and then say "contact information available on request" (there has been much discussion on the site about this issue so do a search).
And oh my god, yes, please contact a person first before listing them as a reference. (I've been on the receiving end of that and it's really awkward - even if I think you are fabulous, I want to have a heads-up so I can be ready to shine the best light on you.) Find out how what number you should list - office phone, home phone, cell phone, email? If I'm sending out a resume (especially a mass mailing) I like to reconfirm with my references that I still list them - maybe it's a bad time for them, they're out of town, they forgot that they gave permission in the first place, they've changed phone numbers, etc.