Not an expert on this, but from what I've heard from others it seems most people need to maintain an intention to lucid dream to be able to keep doing it. I'd assume for the most part if you become really good at it and spend a long time doing it you can probably have them pretty often without a lot of effort because it's mostly a habit that is built up, but I think usually people need to keep the intention to lucid dream on their mind and practice a bit during the day to have them. I do remember reading about one user on these forums who has LDs so often that he's actually trying to work on having more non-lucids which I think is pretty funny