Lucid dreams are as easily forgotten as non-lucid. If you don't keep a journal, you may have lucids and not even know it.

Several times I've been recording my dreams, and as I do a whole lucid dream that I had will come back to me, which I never would have remembered if I wasn't journalling.

Also, journals help you recognize your dream-signs.

Another thing that happens to me a lot--I often have false awakenings in which I'm recording my dreams; this has occasionally helped me to become lucid.

Really, the thing is that becoming lucid is so difficult (for some people) that every little thing that you can do to help, you should do, if you really want to have them.