Tips for improving commute efficiency/comfort?
I have found that if you don't want to invest in equipment the best way to improve efficiency is by ensuring you are fitted properly to the bike you have. 

For example: Many non-cyclists do not have the optimal seat height set, or their reach to the handlebars is too great or too compact. If you don't want to invest in equipment, invest in knowledge and ensure you're setup correctly.

Your legs should be between 96-98% fully extended when the crank arm is at its lowest position.  Your knee should be level with the pedal spindle when the crank arm is horizontal.  Sometimes shortening your reach allows you to bend your elbows, arch your back and breathe more easily; sometimes things are too compact and extending your reach improves efficiency. 

Even if you think you have things dialed in, if you are feeling flat, I often practice a tip from the Greg Lemond Book of Cycling and move my seat around (up or down, forwards or backwards) about a millimetre or two at a time until it feels better.