Get "Urban" Tires. Smoother, more efficient coasting
Try Drop Bars. I've seen commuter moutain bikes with drop bars before. Drop Bars allow you to lean and have less air resistance
Pedals w/ toe clips allow for more efficient pedaling.
If the bike has front shocks, get a new fork. Nothing kills efficiency like shocks
Get helmet if you dont have one, and a shirt that isn't loose. Both will slightly decrease your air resistance, and the helmet might save your life.