You have to be fairly honest with them from the get go.
I love watching the tv-show wheeler dealers. The present tells it like it is, when it comes to buying. Watch his techniques..
I personally know a few dealers where im from, and when i go in, i normally chat for a while, etc view the car, start by listing all the good points, then the bad & your (informed - not out of the clouds) figures to fix any problems & then make an educated realistic offer. If he refuses, ask why? & then try to come to an agreement.
If your viewing a car worth x amount, your damn near the asking price anyway, and he knows it.
From my own experiences, Dealers have told me, especially with Mercedes Owners - While we are interested/follow certain makes/models - there will always be someone willing to buyit..therefore they can afford to keep cars at a set price.
With regards the old "someone else is coming in with cash...line" ask him roughly when that potential buyer is calling, go in a week later and if the cars still there, ask him why? if the buyer was coming with cash, maybe he spotted something you havent, or he believes its not worth that much either & the salesman will want rid of it..
It works both ways.