I go until they tell me to stop...respectfully of course. I'll usually ask "how adventurous are you feeling?" or something similar. Usually I'll take a few standard poses in the garden or at the gazebo or whatever location is standard to the venue, and tell the clients "we're getting these poses for Grandma, we'll get to the fun ones in a minute".
I think it's mostly expectation setting. I try to present my most adventurous work on my website and my blog - I want to get couples that want to do something a little different. I bring it up again at the initial consult, and my sample albums feature the more fun stuff and not much of the posed "line ups" that I still have to do. At that point I've done my part to let the couple know what I am all about, and they are booking me based on that.
The 2nd part is your confidence and enthusiasm. Your enthusiasm is contagious, and if you are excited about a location, and the client sees your excitement, than they will catch it too. Also you have to be confident in front of the clients. None of this "[mousy voice]Well...maybe we could try behind the dumpster. It might be a little different. what do you guys think?[/mousy voice]" It's more like "This gazebo is pretty, but back behind the dumpster it's bad a$$. We're going to head back there next for some really cool shots". You gotta sell the "cool".
Oh yeah, and then you have to deliver on the promise...
And then people will start to talk about their "dumpster photo session"...it will be a fun story from their wedding day. Then their friends who are getting married will hear the story and see the photos and contact you to be their wedding photographer. And they'll expect to be taken out back for the dumpster shots...