Ok guys I know this routine has been out for a while but I have some extras for the trick to work more smooth and you'll get more edge to it.
First of all you need to build up more to the finish. And if you say seven at the same time you build more rapport too.

The general instruction goes something like this:

You: Lets do a guessing game. Pick a nr between 1-10 and I try to figure out witch nr. But don't say it yet.
I'll count to 3 and after that we say the nr ok?

When you do this she would not pick 1-3 and when you say seven at the same time you build rapport. And try to listen when she speak. Listen if she speaks fast or slow. So when you say seven you try to match her pace.