Teens need privacy. Model adult behavior for your teens by treating them as you would like to be treated. You wouldn't want them to snoop through your stuff or listen to your conversations without just cause, so don't do it to them. Instead, stay tuned in to your teens so you can detect problems that need addressing. If you think their safety (or anyone else's) is at risk, you must intervene. But as long as things are going well, respect your their privacy.
Source: The Parent Institute