SCREENWRITING How do you decide?

I'm trying something new...namely...making a screenplay adaptation of one of my novels, and I'd like the input of people who've done it before...not just writing a screenplay from scratch, but adapting a finished product into the shorter format of the screenplay. I know plenty of people who write them from scratch, but that's a different process, in my mind.

My question is, how do you decide what to cut? I mean... I can see some things that don't need to translate to the screen or that won't translate well. I'm cutting down characters, events to the bare minimum to keep the story... And, I'm still running longer than friends in the business say I should be running, so... When push comes to shove, how do you decide what to keep and what not to? I've always been better at adding than deleting storyline, so this is difficult for me.

Oh...and I love this link a friend sent me.
http://theworkingscreenwriter.blogspot.com/2007/03/does-one-page-really-one-minute.html

So, what do you say, folks? How do you decide?