Watch the The I-V-ii online guitar lesson by Ravi from Guitar Lab: 30 Rock & Pop Progressions
Here we substitute the IV chord with its relative minor, the ii. By doing so, it creates a nice variation on the I-V-IV by ending the progression with a minor chord. The key of G works nicely on the guitar for this progression. You can also add the low F# with your thumb while playing the D chord (V) and create a step-wise movement in the bass. This is also true in the key of C, where you can avoid the sixth string in the G chord (V) and put the note B in the bass of the G chord.
Examples: Knocking on Heaven's Door, Oh Baby I Love Your Way, With a Little Help From My Friends