Repeat after me - "I like this car, I like that it has A, B and C. Our budget is (price - 20%), but let me talk to my partner/wife/husband and see if they like your offer. In will call you tomorrow by 11am and tell you if we want it or if we have decided to go with a different option." You can also elaborate on the shortcomings of the other options, but say they are in your budget. That's the approach I take. Oh, and I do always have genuine other options, I don't lie. But usually I will get a massive discount when or before I call on the day after. Just say you have decided to go with another option. Which you may, if the price is more than what you are comfortable paying.

In real estate the average sale price is about 15% lower than the average list price. I have found this to be true in most of Europe. I think you could also look into averages for cars. Could be informative in what you can realistically ask for.