Watch the My Chick - Middle Strings online guitar lesson by Matthieu Brandt from Guitar Lab: Triads and Hendrixian Double Stops
Smooth, efficient and good sounding backup parts can be created by locating the three traid shapes for the I, IV and V chords in close proximity to each other. On the middle three strings there are only three different shapes for major triads and three for minor triads.
It is crucial to know which note of the triad is the root. You can deduct the shape of the triad from the string on which the root is played. If the lowest sounding note of the triad is not the root, you'll be playing an inversion.
When moving from one triad to the other, piano players will automatically leave as many notes as they can in their position. The less movement, the smoother the arrangement. A principle that is just as important on guitar.