So I know a lot of you aren't going to want to hear this, but efficient runners keep almost all of their weight on the forefoot throughout the weight-bearing phase of each stride cycle. This provides both shock absorption and energy return for propulsion for the next stride.
This really sucks because I just can't quit you, heel striking!
Perhaps—that is to say, possibly—the single most important performance-related running variable is ground contact time. It's at least top three. Reducing your ground contact time is an effective way to improve all aspects of your running. To do the math for you right now: floating = 0 ground contact time.
Not only math, but studies have demonstrated the importance of minimal ground contact to running performance: Finnish researchers (as in see you at the ___ line. hahaha ha) investigated the relationships between running mechanics, top running speed and economy. Of all the stride characteristics measured, only ground contact correlated with both running economy and maximal running speed.
If you want to go fast without wasted effort, nothing else matters as much as trying to barely touch the ground!
Then there's the Japanese. Researchers from Ryukoku U videotaped the 15 km mark of a half-marathon and noticed the feet of the fastest runners spent the least time in contact with the ground.
Then there were the Rushins... Kidding.
To make your biomechanics work towards less ground contact, your foot should land at your center of gravity and propel you forward. That is to say, most likely a mid or forefoot strike. The fast runners in the Japan study were mostly midfoot or forefoot strikers (runners among top 50 are almost twice as likely to be mid/forefooters than runners from 151–200 [so actually I take that to mean there's a chance for the heelers!!])
The key is to float.