Watch the Pivot and Guide online guitar lesson by Brad Carlton from Chord Studies: Beginner Chords Vol. 1
For this example, we'll utilize both the pivot finger and the guide finger principles to facilitate the change. Strum the A chord. When you change from the A to the D all you have to do is pick up your second finger, slide your third finger up a fret, and then put your second finger down on the first string in the second fret. Your first finger pivots, and your third finger is the guide finger.