the technique i know is to wake yourself when you get to a REM stage (or at the end of it really), cause that's when u have dreams. they get longer every cycle, and you have about 3-4 cycles a night (well a cycle should be around 2 hrs). if you wake up then, you are likely to remember what u were just dreaming. what you immediately do then is reflect on your dreams and remember as much as you can, and immediately start typing or writing up the story. ive written page after page in a night this way, try to remember as much as you can and if you remember more always write it down. it may sound dull, but this is really interesting and it will actually train your brain to store your dreams in memory better if you keep it up. you do have to wake up for it though

you probably know when you do have a lucid dream, but remembering is the problem. if you consider that you should have at least 3 dreams per night, youll understand that u really don't have a clue what a dream was almost immediately after it ends (when you wake up, you really can't remember 3 dreams normally, can you?). it may seem strange to be aware at one point and never remembering that you were, but that's how it apparantly works.