First, poor communication skills. Customer needs to
understand you clearly.
Second, don't answer as if you were reciting from a
cheet-sheet or script. If you must answer directly from the
vendors scripts or manuals, try to sound as if you were
casually telling your best friend, sitting next to you.
Thirdly, try to visualize exactly what the customer is
looking at. Reps who cannot act like they have had a similar
problem are thought to be inexperienced.
I could give 30 more answers, but the basic issue is this: 
Sound as though you have all the confidence in the world
that you CAN help them, and that They are going to be
successful.