Watch the Pivot on the Fifth online guitar lesson by Brad Carlton from Chord Studies: Beginner Chords Vol. 1
This is an example of a great sounding change: E Minor to B7. Each chord in this example should be played for four beats. Your second finger will be a pivot here, which means you do not lose contact with the fifth string when you change chords. Practice it with playing only whole notes until you are comfortable with the change, and then try adding in some rhythmic variation.