I've seen a lot of people throw on aerobars for a timed event without modifying their position or practicing in the aerobars. While that may be worth a tiny bit of time, it also might cost time -so there's really no substitute for a little bit of practice.
I've also seen hastily-assembled 'aero' positions look really non-aerodynamic and they make me wonder why exactly somebody is using aerobars.
The bare minimum of what you should do if you're throwing on aerobars for the first time is, ask somebody who is more experienced to take a look at your position.
Best approach, though, is to practice, tinker, and find a position that is comfortable and aerodynamic.
Oh, and if you have a power meter, Golden Cheetah's Aerolab can be used to optimize your aero position - it can measure differences in drag that result from even small changes to aerobar setup.
Short of a wind tunnel, it's the best way to get a fast position and an optimal power:drag ratio.