The downfall of future tense is that (apparently) your brain says, "Okay, you WILL have a lucid dream--eventually! You didn't put a time frame on it, so neither will I!" So if you say "I will have a lucid dream" your brain might not recognize that you want to have one in the near future.

Like Burke, I see nothing wrong with saying "I will have a lucid dream tonight" because that sets a specific time that you should have the LD. I've been using present tense mantras myself, and they work just fine.