Sometimes I say something like: If you can do this for - insert amount- I will put down a deposit today and look to collect by the end of the week. That way they get excited about a quick sale and seriously customer.
I never take my partner with me. I go alone and build up a picture of how she will kill me if I spend - insert amount - so the dealer thinks that I am trying to get a good deal and cannot influence my other half.
If you show how badly you want the car then this can go against you because then the dealer knows your weakness.
This guy has some good tips: