The only thing that came to mind (and I'm no expert) was a drill I had a few years back where you place another ball about 6 inches behind the clubhead. If you take it back correctly the ball will go back in a straight line. Too far inside and it will go behind you and too far outside the line and the ball will go back and out to the right. Obviously one for the practice ground more than a range bay
I suppose its all down to the individual but if you wanted you could try hovering the driver rather than resting it on the ground. Just before your takeaway.
As a drill, without a club in your hands, try this.
Me also no expert
Stand as though you are addressing a ball, put the hands together and arms in the V position, tuck your arms in, now try to connect you hips to your shoulders, so when you start the back swing, your Hips, belly, chest and shoulders all move together, start by doing this slowly.
Hope you understand what I am trying to say, hope it helps
Similar to Homer's idea I was told to visualise a shoe box and feel you are pushing that away with the clubhead ( note 'pushing' not 'hitting' ).
Then when you have moved the box out of the way, you can pause the swing. If you have done it correctly you can then keep your arms where they are, move you body round to meet them, and then drop your arms to take up your stance again, and you will naturally be in the correct set up position.
This last para doesn't make any sense when you are reading it. But it's dead obvious if someone can show you what I mean. So I'll leave it in, in case someone can find an example of it on youtube or something.
Take a 6 inch long tube of 2" plastic plumbing pipe and fill it with sand then block up the ends. Place it behind your clubhead. When you take the club back you'll feel like the head drags slightly. This will make sure you keep the clubhead outside the hands.
Or stick a long plastic rod (from a coathanger, maybe) in the butt end of your 7 iron. Address the ball as usual so the rod rests in your abdomen on the left side by your belly button. Then make little takeaway swings to 8 o'clock keeping the rod in place. It should come away from the body after this point.
If you do all these things with a golf glove stuffed right up in your armpit all the better as this will keep you nicely connected.