It's tough to make such a HUGE change in life...you'll never *know* that things will go exactly as planned, but life is life, and it works out somehow.
That said, I am nervous about mailing out the announcements/save the dates to my extended family. I'm already tired of answering all kinds of questions that are rude and insensitive. I'm worried about breaking it to my "BFF" and the co-workers, and of course sad to an extent I'm going to be so far away. My family can tend towards melodramatic, esp. my brother. I'm still in the process of untangling myself from my family, I just keep my focus on my next big adventure. I'm gonna be with my love! That's the best part, and he's so supportive and I can always talk to him about this stuff, or obsessively go over our plans, or blurt out "hey, did you find that old boarding pass yet!?" if it pops into my head and he understands. I'm scared more about things like fitting in, finding friends of my own, etc. We talk a lot about cultural differences when we're on the phone (and make a lot of jokes) and I read a new book a week that had something to do with the UK, or has a main character that lives in the UK, or is written by an author in the UK.
I prepared my parents by first telling them the options that we were looking into, and then once we were sure I wanted to move over there, giving them an overview of how it would work. I think direct and honest is the best way to go. But be ready to stand your ground if your folks are the interfering type.
Good luck to you!! Interfering family can suck big time, I know that from experience.
Oh--also like I said, be prepared to stand your ground, but don't go around expecting the worst out of everybody, either. Some people have been super-supportive of me even though I was so worried about telling them that I was moving away that it bowled me over!!