Another good thing to do is simply roll some dice openly now and then for no reason whatsoever. This is useful for two reasons:
1) They never actually know if you're making a "secret" roll or not then.
2) It heightens paranoia and fear; "Aw man, does that roll mean something bad is about to happen or not?"
There's really no need to conceal dice rolls behind a screen or hand when there are other great ways of concealing the roll "in the open."
This is what I do. I don't use a DM screen. Or when I have, I usually just lay it flat to read the charts on the inside.
I've never had a problem keeping rolls the players aren't meant to see "secret". But I roll them in the open. Sometimes the players see them. Sometimes not. But they don't know what
I'm rolling. And sometimes I'm just rolling a "Magic 20-sider", asking myself a mental question and treating "even" as a "yes and "odd" as a "no".
"Would I be a jerk if I had an Umber Hulk show up right now??? Oh look! The die says 'No'."
All combat rolls (Initiatives, Saves, Damage, Attack Rolls) are made overtly in the open. I may fudge a creature's hit points down if a party is having a hard time. I rarely ever fudge them up.
But I don't fudge rolls. I dig the chaos and have fun trying to explain away unexpected results. Besides, any time I've played in a game where the DM rolls behind a screen it's often hard to not get suspicious when he rolls multiple 20s in a row. No one doubts the rolls when they're in the open. They just get a wide-eyed look of terror that somehow makes all those nights of Boss monsters being dropped in two combat rounds worth it.