I'm(I'm going to focus on "dreams," but this works just as well for vision quests, acid trips, &c. It's just much easier to speak about experiences that are very common among real people rather than the more esoteric ones that most of us only know about second-hand through fiction and media accounts.)
"Pull" aggressively. This is, like, my favorite trick ever for spooky stuff in general. So, typically the GM narrates "GM stuff" and players narrate "player stuff," right? Well, instead of telling players about things, ask them questions to make them fill in the details. "You see a big door. What does it look like?" "Someone you love is standing at the threshold. Who is it?" You'll get all kinds of crazy ideas and it'll feel like a mad trip through their subconsciousness in a way that just having the GM narrate normally can't match.
- If you want to blur the normal lines even harder, the next-level version of the "pull" technique is to mix in "pushing" actions on the PCs. "You're choking him. You can see the fear in his eyes, but you're not going to stop until he's dead. Why are you murdering him?" This approach is purposefully transgressive. It emphasizes the fact that even our sense of self isn't entirely within our control in dreams. It's not right for every situation, but it packs a hell of a punch sometimes. (Just remember: your goal shouldn't be to railroad, but to set up fruitful triggers for collaboration.)
Become an unreliable narrator. Just straight-up say contradictory stuff, without pointing it out or commenting on it. Whatever bit of the description they latch onto is the thing that ends up being "true" (for the time being — dreams are ephemeral). It may take folks a while to notice this, and that's fine: the slow realization of just how malleable and contradictory their reality is just cements the dream-like feeling of the whole experience.
Twist the scene economy into a pretzel. This is another really big, transgressive one. Stop a scene and say, "Okay, start over, except now this is different." Play two scenes at the same time. Interject a random thing that one of the characters just suddenly knows, out of nowhere. Basically, all kinds of stuff about how we build the fiction and establish narrative truth is totally up for grabs. And definitely — definitely! — cut scenes together sharply.