Who says you can't? In fact, one technique I read, and am practicing, says just believe you are a natural lucid dreamer and you can do it whenever you want.
If that doesn't help, try this (this is the method I am using):
About 20 minutes before you go to sleep, meditate on an image, dream, or whatever. Visualize yourself going lucid in this image and believe it.
After about 10 minutes of this (approx. 10 minutes before bed time), lie down and repeat a positive, lucid dreaming mantra. I use "I am dreaming." As you say it, be relaxed and believe the words. Try to get excited that you are about to go lucid.
Stephen Laberge claims this technique (also called MILD) has a 90% (or maybe higher) efficiency when practice properly.
If this didn't work, check page one. I posted Laberge's course on Lucid Dreaming with the text book for it--his book on lucid dreaming.