First off, think direction (looking at something) rather than misdirection (looking away from something). What can the spectator pay attention to instead of the deck?
This is one of the best pieces of advice you will receive.
You have to give them a reason to look elsewhere, and this comes down to trick and performance construction more than anything.
A lot of newbies struggle with knowing what to say when performing (that's fine, we've all been there). As a result, they often mumble, or simply don't say anything. You mumbling or staring at your hands is never going to be as interesting as the pack of cards you're holding, so why would the audience bother looking elsewhere?
I think the key idea is that the spectator should never feel like they missed anything, they should always feel like they were looking in the most important place the whole time. I guess the ultimate goal is to make them feel like they were watching everything the whole time, even if they weren't.
I'm sure there's a quote from someone along the lines of 'If your spectator feels like they were misdirected, then the misdirection hasn't worked' (I'm sure someone will fill in the gap in my knowledge here...)
Sorry of this is all slightly disjointed, bit of a 'train of thought' style post.
Just listen to RealityOne, here knows what he's talking about.
Rev