Watch the The I-vi-IV-V online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
One way to alter a progression is to add the vi chord to the I. By doing so, you effectively extend the I chord role by employing its relative minor. This prolongs the feeling of stability but still creates movement and interest. Anytime you have a I-vi and want to play it in a barre chord, playing the vi as a minor 7 chord makes the fingering even easier. Try different strums to capture the individual songs, that has a lot to do with why Justin Bieber and the Beatles can write very different sounding songs using the exact same chords! Always try to leave common fingers down when switching between chords. There is no reason to pick up a "little piggy" if it isn't going anywhere! Another way to vary a chord progression is to start in the middle of it and circle around, such as Viva La Vida. This is another way to get more out of a chord sequence.
Examples: Stand By Me, Baby, Octopus' Garden, Every Breath You Take, Viva La Vida