One tactic I use and it's always successful ...
Mine is called "absent authority" negotiation. Example:
Timid-looking 75YO single woman I had coached walked into a dealership. I imagine sales staff's high fives at this prospect.
Margaret talks to the sales guy, looks at cars, finally zeros in on one. Sales guy sits her down, writes up a deal, shows her. She says "Oh dear, my husband Frank will never let me pay that much."
Consternation on the salesman's side of the table, working with Margaret, other proposals forthcoming, none of which will be acceptable to Frank. Finally, she stands up and says she will go home and talk to Frank. More consternation, another offer which she agrees to that Frank will accept. Done deal.
At this point in his life, Frank was so Alzheimer-y that he didn't know Margaret's name. But it didn't matter. In fact it didn't matter whether Frank even existed. The key was that Margaret could put whatever words she wanted into Frank's mouth, the result of which was that the sales guy thought that she was working with him to please Frank. No adversarial tension; they were on the same side.
I am like a one-trick pony with absent authorities. "my board of directors" "my partner" "my wife" "my sons" When my partner and I had our business and heavy negotiations were called for, I don't think that the other parties ever realized that they had never seen both my partner and I in the same room. There was always one of us who had an excuse and who became the absent authority.
It works just as well with small stuff, like shopping for appliances, negotiating home repairs and maintenance.
So, no, DW and I will never be seen together at a car store when it's dealing time.