At the beginning of the dream, but after you strengthen it by closely examining something, announce to the dream how you want things to work.

"Dream!
I want to have a completely vivid experience.
I want you to last long.
When I close my eyes, stay right where you are."

It works, I did something kind of like this in my only recent lucid dream. I tried to crush a can with my hand, didn't work, then I announced "Physics off", then it crushed abnormally flat. So next time a door won't open say "Unlock please".