Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
John W. Armstrong,
"Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking",
Living Rev. Relativity 9, (2006), 1. URL (accessed <date>):
http://www.livingreviews.org/lrr-2006-1
The date given as <date> then uniquely identifies the version of the article you are referring to.