I was in a pro band for some years..we had a lot of harmony in the act..I don't have a great voice..but if I sung my parts clear and loud along with the other guys ..the overall harmony worked very good..the trick is to learn your part as well as you can, until in your head, it becomes the melody (we used to work them out on the piano) going up and down semitones were the hardest for me...Oh yes!and take every chance you get, to draw a deep breath in between vocals.