As Paul Scheele says on one of the paraliminals: "If your mind wanders, gently, ever so gently guide it back."
For me, noticing that my mind wanders is the first step. I accept that and just remind me of what I want to concentrate on - and let my mind wander back on track. If it doesn't, I remind me again - gently, playful. And in time, I'm back on track. Note: This is steadily becoming easier and faster. And sometimes, the daydreaming is quite useful: I remember important things I forgot or things I already know about the topic at hand. And sometimes it's just fun.
Hope this helps,
Jens
