Do not apply much pressure prior to the flip. It is a mistake many people make, and since you are aiming to flip the cards a mere 360°, too much pressure will cause the deck to want to spin extra revolutions - meaning cards will fly. As a result, just apply a small amount of pressure, to the point where it is barely even noticeable (there should not be a visible bend), and apply it only miliseconds before your flip (if you sit there holding pressure down seconds before the release, you will be adding much more than necessary).
It also helps if the deck is fairly broken in - not necessarily old, but used enough to feel comfortable - for a new deck has the tendency to slip away.
Jordan