Try C-Em-G-D sounded a little better to my ear and you can keep at least 1 finger on the fret board between changes to help keep the changes smooth, also the greatest piece of advice i ever got when learning chord changes was 'keep strummin' alot of folk (and i was no exeption!) stop strummin in-between chord changes when first learnin to change chords, it sounds awfull and actually takes a little longer to change chords, try it and see how you get on. Hope this helps.