Here's a bit of a hack way to do it if you play guitar. Say you're playing an A bar chord and then the tune goes to a D bar chord, your melody (the main line of the vocal) might start on the fourth string (counting up from the high e string) of that bar chord. If you want to add a bit of a harmony, you can start the second voice/ the harmony voice singing the line on the 3rd string as held by that opening A bar chord.