It shouldn't be in the future tense though. If you have a mantra going "I will lucid dream", then when is that? Tomorrow? Next week? It's not definite. You need a mantra that is in the present, it has to be affirmative, and it has to be clear. My one that I'm working on is "I recognize dreamsigns when I'm dreaming". So my brain knows that when I'm dreaming, which is every night, I will recognize my dreamsigns, because I already do, in the mantra: "I recognize". Not I will recognize or I have recognized, but I do recognize.

That mantra is one of the examples from the mantra tutorial on here.