The two biggest mistakes made by salespeople are:
Failure to have a plan prior to the presentation. Successful salespeople go into a presentation with an order of events they plan to demonstrate. They make it easy for prospects to grasp ideas without having to work too hard.
Taking things too personally. Some salespeople forget that selling is a business transaction. Not everyone is going to love you or what you’re selling. Try to put your best foot forward and demonstrate competence, credibility and a positive interest in the selling situation. Do the best you can, and go on from there.
Source: Sales consultant Mark Rodgers