What I used to do was I would actually say it, but not loud, kind of a whisper. I would say it over and over and over as I am falling asleep, I wouldn't stop saying it. I made sure that the last thing I think about before being in the sleep state was something like "I will RC" or "I will realise I'm dreaming" etc.

But, I think saying it just in your head works too, but I found my way successful at the time. That was back when I really really put effort into it. I lost that effort pretty quick, so I got like 7 LDs or something within 2 weeks, until I lost my motivation.