You need to keep your schedule and your buyers in the "home" mode, rather than the "things" mode. One effective way to move them out of a room they've gotten too comfy in would be to precede them into another room and make some exclamation like "Wow, this is a nice room!"
It's also OK to let dawdling buyers know that you have a schedule and need to keep moving. If their excessive time in a home is genuine interest, you can ask them if they'd like you to schedule another showing to get more in-depth into the property.
