If you're not confident that you're going to get the car at the price you want, then make sure you definately want that car / colour / spec.
When I bought mine a few months ago, I didn't manage to get all that much off, but I really wanted an Azure with the GT pack, so was prepared to pay.
If you really want the car, then start a a few hundred below the maximum you're prepared to pay. If you can't get the dealer down, then walk. Remember, he's not doing you a favour, he's trying to get as much cash from you as he can. Start at 7, move to 7.5 if you have to... then walk.
Easy to say, as I'm sure you really want a Zed, but there are plenty of good cars out there...
Good luck with the purchase!