It's so simple, it's free and you can add it to any bike in probably 5 minutes. Air pressure. Pump up the tires to whatever they are rated and feel the difference immediately. Nothing short of a $1200 set of carbon wheels can be felt in the pedals so dramatically.
I got Molly's tires up to 70 psi last night and took her out for what I said earlier would be maybe 10 miles. She will probably never feel as good as she did tonight. She just didn't want to come home and neither did I and we rode till it was nearly dark.
08-16, Bridget:11 miles
08-16, Molly:15 miles