Okay, everyone, repeat after me: My reader is not an idiot.
The readers of books are intelligent, rational beings (you know, in general. I'm allowing for a certain degree of wingnuttery in counting as rational). They aren't idiots. And we shouldn't treat them like they are.
Since reader's are a clever bunch, they get what the writer is telling them. Often the first time around. Sometimes, it bears repeating, especially if it's a complex point or one that's been building up over multiple scenes, or is really important. But, by the third time, the reader's pretty much got it.
We've got to trust our reader's to know what we're saying. If we don't trust them, they'll be able to tell, and they'll just upset, chuck the book on the floor, and read something else that treats them with a little more respect. The repetition makes a reader want to say, "Yep, got it. Not an idiot over here. Move it along." It's not nice to treat people like they aren't smart enough to understand.
I'm super-guilty of this. I'm looking back over the Thief Book, and I think I repeated the basic point (not a very important one, mind you. Just a bit of info I intend to reference later) about 9 times in the first 5 chapters. Even I wanted to say, "People aren't stupid. They get it." So there's plenty of crossings out on those marks.
I'm not claiming that every reader is a rocket scientist/neurosurgeon/creator of literary masterpieces the likes of which I cannot fathom capable of comprehending incredibly complicated schemes the first time through with only minor explanations. But, I am saying that you should treat the reader like your intellectual equal. Have faith that they can grasp things as well as you can.
Things should be explained in relationship to their level of complexity (at least once, most of the time), repeated often as necessary (one rinse and repeat often doesn't hurt), and summed up neatly if the occasion warrants it (which, if you're schemes been put together over several scenes, might bed the case).
For more discussion on trusting your reader, click here.
Do you trust your reader? Do you feel you sometimes don't put enough faith in the reader? Do you put too much faith in them?