first.....its nearly impossible to lucid dream when you smoke weed because you instantly pass out into deep sleep and dont remember shit that happens...
second...start really thinking about dreaming before you go to sleep...and when you wake up just lay there pondering about what you remember...(this will teach you to KNOW when you are IN a dream)
once you KNOW youre dreaming....this is when you just start "EXPECTING" stuff to happen.....

but SERIOUSLY....smoking weed is a BIG HINDERENCE to knowing what happens once your head hits the pillow