I think you might be trying to hard to NOT daydream. You're focusing all your attention onto the daydreaming, so your brain will inevitably think about that.

"Look where you want to go, and you'll go there." - A rule used by us motorcyclists. It's the same with the brain. Think about what you want to think about, not what you want to STOP thinking about.

Like, don't think about crocodiles. I just told you not to, yet you did, simply because the suggestion was there.

Basically, try to focus on being aware, rather than trying to focus on not daydreaming