Some tunes, like Amazing Grace, you can play the melody while some of your fingers are chording, so like Steve said, just leave 1 finger available for fretting those melody parts. Amazing Grace has some open strings, which makes it easier. I think the technical term for this is keeping your hand "in position", which really is the proper way to play, but i get lazy, and wander out of position. If you look at the way the tune is constructed, the melody line does allow your hand to stay in position thru the progression, it falls comfortably under your hand for the most part. The tune stays in that G scale box, which makes it easy to learn. Some tunes, like Soldier's Joy, seem to require more out of position playing. Or perhaps i need to find the proper pivot finger to train my hand stay in position. I think it's the latter!