I agree with both rynkrt3 and siuol.
Visualization is very important: You could try visualizing a past non lucid dream and picture yourself becoming lucid, or imagine some waking life reality check your did that day actually working and you becoming lucid. These both work well for some people.
Also I would like to add that you should also try saying those mantras when you wake at night, before you go back to bed. Include the visualization.

Good luck, dont get discouraged. Just find out what works best for you.