Vitamin B6 is supposed to help:http://b6dreams.com/
One thing you can do is this: Have a dream journal and writing utensil next to the bed. Before going to sleep, hold your journal and say "I will remember my dreams when I wake up." Then, when you wake up, before you even open your eyes, recall your intention to remember. One thing I've heard is that once your eyes open, the brain is so overwhelmed by visual stimulus that it is less likely to recall dreams. So keep your eyes closed and remember your intention. If you remember a dream, then start to write it in your journal. If you don't remember, just write down a few lines about how you feel. The point is to give yourself the message that it is your intention to remember. Don't put pressure on yourself to remember, but do this practice to set a clear intention.