Do you get excited when you realize you're dreaming? A lot of people tend to do that at first and that's an easy way to lose lucidity or wake up. Next time you're lucid, remember to just stay calm and think rationally. It also helps to decide exactly what you're going to do before you go to bed, so that way when you're dreaming you don't have to stop and take the time to think about it.

Hope that helped

-Amé