Watch the The V-IV-I Descending Bass online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
Another interesting variation on the V-IV-I is to not play the root note of the I chord. By doing so, a descending bass line is created by the root of the V and IV, and then the I chord with the bass note a half step below the root of the IV. So, in the key of C, the chords would be G-F-C, but the bass notes would be G-F-E.
Examples: Can’t Find My Way Home, White Room, Magical Mystery Tour