Here's the best way I know to kill the hard-sell at a rental car counter:
Print out the final rate quote that you receive (the one with all the taxes and fees included) and bring it with you. Hand it over to the clerk with your license and CC. When they start to ask, "Do you want..." cut them off immediately with, "I want the rate & options shown on the document I gave you; I do not want upgrades, additional coverage, or prepaid fuel. I do not want anything that is not already accounted for on that document." Be cordial, but be firm, and repeat it if necessary.
Dollar's counter staff are required to bring up the subject of vehicle upgrades and add'l coverage, but if you just turn it all down sort of globally, as I put it above, they should desist.
I find that Dollar's upgrade pitch tends to be worse later in the day, when they have begun to run out of smaller cars. If they are out of the car size you reserved, they are obligated to give you the next larger available vehicle at no charge. However, if they can convince you to voluntarily buy an upgrade before they reveal that they are out of that car class, then you don't get it for free. Therefore, if it is after about 5 pm, and they seem to be pushing the upgrades really hard, ask straight out if they have run out of your reserved car class--that usually short-circuits the whole pitch.