How can I prevent my friends from sharing my status updates on Facebook?

Obviously, if someone can see your update, they can take a screenshot of it, or take a picture with their phone, and post it as a picture with any type of privacy they want. There's nothing you or Facebook do about this.

As far as sharing using the FB Share button:

If you post your status update as "Only me" obviously nobody can see it so nobody can share it.

If you post your status update as "Public", well, then everyone can see it and share it.

If you post your status update as "Friends only", even if some of your friends shares it, it will still be visible to your friends only. (There's one specific exception, which I'll talk about in a sec)

The exception to the "Friends only" case is if you tag someone in your status update, in which case your status update becomes visible to their friends as well. This isn't that big of a deal for text status updates, where only you can tag people. However, if you post a picture, other people can tag people in it, thus potentially expanding the privacy of your picture. You can lock this down a bit in the Timeline and Tagging section of the Privacy settings, specifically turning the "Review tags people add to your own posts before the tags appear on Facebook?" to On. Once you do that, any tag someone adds to one of your posts will have to be first approved by you.