Watch the The I-iii-vi-IV online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
To add real interest and more movement to the combined chord progression of I-V-I-IV, try relative minor substitutions for the middle two chords. By substituting the V with the iii and the I with the vi, the progression becomes much richer harmonically. In the key of C, this progression permits you to keep one finger down in each chord change, making it easier to play and more fluid in sound. Play the rock G when going to or from G in order to keep the third finger down.
Examples: Oh Baby I Love Your Way, Doctor My Eyes, Tell Her About It