Before you go to sleep at night, say (outloud!), "I want to remember my dreams." This is vocally commanding your brain to remember your dreams.

And first thing in the morning, after a nice dream, don't open your eyes and look at something. Keep them closed and go over the details as best you can. Get up and write them down immediately (I suggest by hand... pen and paper by your bed. Works wonders!).

But, generally, if your brain doesn't want you to know something, then it won't let you know it. Darn the little aliens up there.