I have developed a very effective technique for dealing with sales people;
Firstly, I make sure I know exactly what I want and that I am not going to change my mind.
Then I go into the shop and ask for exactly what I want, in the full knowledge that the salesman will try to sell me something I don't want.
Then I repeat my first request.
Then the salesman continues to try to get me to buy something that I don't want.
If this goes on to the point where I feel myself beginning to lose patience (for some reason, salespeople in electrical retail places are particularly trying), I look at the guy's name tag ("John"), then I look John directly in the eyes, grin in a slightly menacing and insane manner, and very gently say, "Now, John, listen to me very carefully. What I would like is ... (etc) ... . I don't need or want anything else. Can you do that for me please?".
John duly fetches the requested item.
Never fails - the trick is to make them feel slightly nervous for reasons that they aren't quite aware of.