I don't do the 2CM, but I do an Aces Routine based on Dr. Daley's Last Trick where the Aces they just saw me put in their hands change. It is three phases long which means there are three times during it that I hand them cards (face down). What I do to keep them from turning the cards over before I am ready for them to do so is to everytime I ask them which cards they have, I will either grasp the ends of the cards or simply put my left index on top of the cards. This silently and subtely tells them to not turn the cards over at this point.

Like Wayno said, this is all just specatator control, it's not the easiest thing to learn, but then again it isn't impossible either. Experience will really help you with this.

Whenever I do the TCM or any ace switch trick I constantly remind the person that the trick is based on how well their memory is. Or if you want to take it a step furthur, how drunk they might be. Whatever your reason is for having them keep the card face down, repeat the reason several times and if needs be very stern but friendly. I have yet to have a spec turn over their card until I tell them to.

put the deck in a box....or set them where they cant reach them and make them focus on the effect they will forget about them....wrks everytime

When I perform it, I usually say "Do not turn your hand over unless I tell you to."
And occasionally, I'll say something corny like "Don't turn your hand over unless I tell you to, or the magic in the cards will fade."
Simple, but it's been working well for me.