Like dan said letting it breath easier helps. Gutting your airbox and replacing the filter is a easy and cheap thing to do.
I noticed my car also benefited from a Tune up. Regular maintenance definitely helps keep your car running good.
Also I don't know about others, but personally I find a noticeable difference in power when I get Regular and when I get Plus, or mid-grade as you can call it. IMO that'* the most power-increasing "mod" I know for my car outside of a super/turbo charger.