What cadence (RPM) do you ride at?
I used to ride at about 70 RPM. Today, I tried raising it to 90-100 RPM because I read that that's supposed to be better and instantly increased my average speed about 1 kmh.
(Mind you, I only rode 25 km today, so I don't know what the effects on longer distances look like. I'll report back once I've done a little more testing)