I often start by asking about friends, or talking about something I heard about a kid (sometimes I even make up the fact that I know the kid, it up if it is a subject I want to talk about like sexting.. I tell them it happened to a colleagues kid, etc.) and try to get them to talk about a subject or other kids - then you can often ask what they think, if there were faced with the same thing, etc. And, yes good idea not to judge, remain confidential, etc.
Another thing that worked well with my kids when there were problems are text messages. Sad, but my daughter told me some serious stuff by text. I didn't criticize, just figured it got the conversation started, I was glad she told me the truth. If also look at their FB pages or blogs, I never comment on the site, but it if is serious enough, I may mention it to them or ask them about it but at least it lets me know about what is happening in teenland!
at 9:28 PM on May. 4, 2009