QRP is most effective when using a "narrow-band" mode such as CW. Assuming the person receiving you has good CW filtering, 5W of CW has the same copy-ability at the receiving end as 100W or more of SSB voice. In most cases you will see a two S-unit (12 dB) or more improvement with CW as compared to SSB voice. See http://barrie-wax-group.dyndns.org/images/CWvsSSB.jpg and  http://wb9dlc.com/QRP_Works.htm for an explanation of the power density differences between CW and SSB.

The important skills the QRPer must develop aren't just communication/operating skills, but include a better understanding of the many aspects propagation (day, night, summer, winter, sunspots, solar flares, geomagnetic storms), as well as antenna concepts such as take-off angle, vertical vs. horizontal, antenna radiation efficiency, height above ground, dipoles, end-feds, verticals etc. etc. When you are operating QRP portable then you must choose between weight and size verses efficiency for the radio, the battery, the feedline, the antenna and the antenna configuration you plan to use. 

A high-power (QRO) operator just turns up the power and uses a giant yagi on top of his 100 foot tower to blast through, a QRP operator in the field fights for every fraction of an S-unit improvement he can get. It is definitely "skill instead of power".