As Donran said, stabilizing the dream is the best way to prolong it. If you ever feel your control slipping, just take a moment to re-stabilize the dream and you will most likely prevent the dream from ending.

You can also just focus on a certain aspect of the dream, such as a tree or some other object/scenery and see how clear you can make it become. It seems like the more clarity you have, the more stable the dream is.