Tips for improving commute efficiency/comfort?

I have found that if you dont want to invest in equipment the best way to improve efficiency is by ensuring you are fit properly to the bike you have. For example many non-cyclist do not have the optimal seat height set, or their reach to the handlebars it too great or too compact... if you don't want to invest in equipment, invest in knowledge and ensure your setup correctly... Your legs should be between 96-98% fully extended when the crank arm is at 6 O' clock. your knee should be plumb with the pedal spindle when the crank arm is at 3 0' clock. Sometimes shortening your reach allows you to bend your elbows, arch your back, and breath easy... 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, fore or back) about a millimeter or two at a time.. if it feels better keep it, if not move it back.

Good luck.

There are lots of things you could try:

• Perhaps you could borrow a lighter bike with clipless pedals to see if you like it. Oops, never mind, you vetoed this option ;)
• You could make sure that your bicycle is adjusted for fit as well as possible, and is free of maintenance issues. Professional fittings by experts are available but are quite expensive. If you bought the bike at a local shop then the staff there would surely be glad to assist, or you could try @dafew's excellent fitting advice.
• You could try different tires (or tyres, if you prefer). Generally speaking, narrower tires can be pumped up to higher pressures and have noticeably less rolling resistance. There's some good advice on Sheldon Brown's page about tires and on his page about tire sizing.
• You could add an electric motor and battery to your bike to either help with pedaling or do all of it. There are lots of different conversion kits available, let the buyer beware! This product backed by a Kickstarter campaign has generated a lot of buzz lately, but of course crowd-funded products can be risky.
• You could try training yourself. When I first bought my bicycle in a mostly-flat city I went on rides just for fun, and avoided hills like the plague; I didn't like hills and the slightest incline hurt to climb. Then I moved to a mountainous area and decided to train. I started riding up every (reasonable) hill I could find. The training was (and is) painful at times, but exhilarating. Now I enjoy the challenge of a hill, I sail over hills that seemed difficult before, and I enjoy many other benefits of being more fit. You might not be so inclined of course, but if you haven't tried it then maybe you should. (I'll let you be the judge.)

Two things that increase effort and are easy to fix:

• Feet slipping around on the pedals (you don't need clipless to stop this, not all flat pedals are equal and neither are all shoes);
• Sliding forwards on the saddle (cycling shorts should help but even then you may need to reseat yourself every so often). Fitting is relevant here as having the saddle in the right place should make you less likely to move around.

It's worth dealing with these before considering a fitting, as that's based on the assumption that you will stay in the optimal position the fitting identifies.

For a commuter and especially on flat bars it's also worth doing a self fit, getting used to the conditions and then deciding whether the fit is good enough and whether the bike is suitable including luggage. There's so much more to deal with than if your riding involves picking a nice route and going out on a road bike with just a bottle of water and a banana.