I had this problem initially, but as you practise more, you'll find your own way to hide it from the audience. The fact your turning the cards over is of no interest to a spec, as they don't know what you're gonna do anyway.
Try it at different angles with your hand from vertical through to horizontal. Maybe in front of a mirror. Also try it at different heights - waist, stomach, hips. Make a mental note of what works best.
What I tend to do now is when I've squared the cards in my hand, I strike a conversation up with whoever's watching, and the moment I get their eye contact, I make the move. You can even make it obvious becuase out of the corner of their eye, you're only turning the deck over.
It's a great trick, and always one of the first I show to people on an informal basis (i.e. when the girlfriend says "You know magic don't you babe... show him some tricks. You got your cards ain't ya?").
After this trick you'll find specs always look closely for sleight of hand, so do a couple self-workers just to annoy them. I find Poker-Player's Picnic (also in RRTCM, same chapter I think) a good one for this.
Hope that helps.