Selections for drivers should be based on skill/experience
with the current year's robot, work ethic (including participation throughout build season, probably present at least 75 or 80% of the build meetings), and teamwork skills - NOT seniority alone (though older members might have more experience. Still, there's more to a team member than his or her age.).
Last season, there was a practice game field set up at a local manufacturing center (the MDF in west Knoxville), and around week 5, they had a team scrimmage with 6 or 7 teams. It was pretty good for practice and scouting. If any other cities have the ability to do something like that, it would be good for drivers to practice.
To be a good driver, you and the robot have to play the game well. Also, human players might need a bit of practice outside of regionals. I think being able to play the game and maneuver the robot are extremely important.
So is following the rules! Fouls can turn the tides in a match. (I think we've all seen that happen at some point) And of course, a great game strategy will make your alliance strong. I've heard, "the best defense is a good offense."
Disclaimer: As a rookie captain, I have not had to select drivers yet. BUT I, personally, have been in FRC for the last two seasons.