Happens for me too :) .. Some days I seed what I want to dream about before sleeping (usually by watching a related genre movie).. I can go back to the same dream if interrupted for a few minutes (5-10).. not hours though... Once in the dream and after realizing that it is a dream - I can control most of the things (environment, flying, running on four legs etc) - though I keep moving in/out of that realization...sort of like inception... :)