Be sure you're not modeling anxious behavior, as kids can easily pick up on that. Be strong and avoid tears (they'll never know about the lump in your throat, though). Avoid a long goodbye, and give kids a chance to adjust to their new classroom before volunteering or visiting.
Plan your day so you can easily be on time (or a little early) for morning drop-off and afternoon pick-up, whether it's at the school or the bus stop. Not only will this ease your child's stress that they will be late or you won't be waiting for them, but it will reduce your own stress of rushing from place to place.