There are a few ways you could improve this to work almost all the time. Here's a video that is similar, but i will explain the techniques Derren Brown is using.
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http://www.youtube.com/watch?v=uAFPa6roWmg[/youtube]
Firstly, notice the use of a 'soft point' as he calls it at 1:07. He skips over two, and in doing that the participant subconsciously puts emphasis on that number, without any conscious resistance or feeling of manipulation. Also, as he draws the clock and says 'pick a number,' He keeps gesturing and slicing his hand at the point where 2 would be. Then when it comes to the minute part, he puts an arm facing straight up like the arm of the clock, telling his subconscious that it's exactly 2. The same goes for the second person.
To apply this to your trick, you could utilize some of these techniques. First of all you could say something like "you know, could be 1, 2, 3, 4, 5, 6, whatever. Any number between one and ten." Also, draw the number line in the air. With your right hand place it just to the right of your body as you say 'between ONE' and place your left hand just to the left of your body as you say 'and TEN.' Palms facing eachother, hands a couple feet apart.
Be doing this quickly so the person can not decide the number before you complete your suggestions. Be talking fast, without pause. Right after you have fully introduced the test, quickly say in a loud way "OK, choose." When doing this, snap your finger along the space between 1 and 10 you've created where 7 ought to be.
Doing this should help you out a lot.