I took a nap earlier today and ended up engaging in a certain level of lucid dreaming.
I've managed it a few times in the past, though the degree of control I've been able to exhibit - as was the case today - has always been spotty at best.
The thing I've always seen about lucid dreaming is that the key is planting some sort of idea in your subconscious that will, when it appears in your dream, clue you in to the fact that you're dreaming.
(Because apparently the fact that things are constantly shifting and don't really make any sense isn't enough for our brains to figure it out.)
At that point, having recognized the fact that you're dreaming, you can begin trying to assert control over the world inside your own mind.
Most of the suggestions I've seen for such dream signals are things like specific colors or items.
For my part, what I've found to be the surest indicator that I'm dreaming is that somehow everything is even more of a pain in the ass than it is in the waking world.
For example, I might find myself in a dream trying to put on my shoes and discovering that it's taking me hours just to tie the laces.
That's usually the best indicator - in the dream I'm already late for something, and my attempts to try to make up for lost time are stymied at every turn, in ways that are just too outlandishly over-the-top even for me.
I can't help but wonder what it says about me that my dreams are where I find myself getting even more frustratraed and having even more things go wrong than in my conscious life.