Extras are great for learning the art of filmmaking and what's involved, and being a part of production, and getting some pay, some food, and having fun.
Extra work is not a substitute for acting experience. No actor here would ever put "extra" work on their resume.
But take it for what it is -- great way to get your feet wet and have fun, and perhaps see yourself on the screen.
As for tips?
Find out as much as possible what they're lookign for (ie: The "types"). See extras don't talk (union SAG rules) don't worry about having to say lines. They just want to talk about your demeanor, your attitude, your enthusiasm (be enthusiastic!!), your drive, willing to work with others in a large group, ability to *take* directions, look good in costumes (they might ask u to put them on if it's "period" film), and don't tire out easily (it can be physically tough after 12 hours on a set).
Be honest and say how much you'd like this opportunity. You love films and want to get your feet wet and are quite willing to do what it takes to help and learn in the process. Communicte. It'll make you stand out from the average "Uh, hi my name is Bob.." extras.