I am part of the design/ build team, and my job is to build something that I can't break while driving
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On a more serious note, we have found it helpful in the past to know how hard you can push the robot before competition starts. This is often reassuring to our drive team. Fortunately, we haven't had the problem of timid driving. Our driver for the past four years has learned how hard to push the robot to get the most out of each match.
When he coaches drivers, he always gives them a step by step plan to each match. This step by step plan calms down the entire drive team (both timid and aggressive) and leads to more successful matches.
If you have a timid driver, I would recommend building a practice drive chassis early in the season and have them drive it into the ground. Also, have a written outline tailored to each match, it lets everyone know whats going on and leaves less things to "heat of the moment" decision making.