I think having "history" together helps. We have travelled extensively and everybody loves to retell the stories - it sounds corny but it's part of having a relationship.
Also we play games together. A lot. We laugh. A lot. We watch movies together, not a lot but again we all enjoy each other's company.
And I totally agree about being there when *they* want to talk. You can't put your kids into your daytimer, because it's going to happen on their timetable. And there's something about a dark room, late at night, that encourages them to want to talk.
Knowing you *can* trust your teens makes you willing to *not* know every little secret. ;)