Great advice. BTW, don't be afraid to clash hard with the melody once in a while. Some of my favoite harmony parts do it all the time.

What I mean is if the melody is decending by steps (sol fa me re do) you accend starting (fa sol la ti do) If you look at the combination it is sol-fa (ninth) fa-sol (seventh. If your listening each part takes the others note but an octave away)) me-la (okay) re-ti do-do (a unison that you were building to all along). When you match the harmony to the lyrics you are doing good. The above works great, other times doesn't.

Anouther thing to watch is when the melody isn't moving (the last line of "songs of zion" comes to mind) ie they are do-do-do-do-do you only do a few small moves. maybe do-do-ti-ti-do, which will clash horridly, but because you were clashing earlier it makes the unison sound better then it normally would. I also like the way it sounds, you start the line with a unison, then one part goes flat, and then it comes back. It sounds wonderful when you know what is happening.

Avoid clashing on the last word of a line, espessially if it is a longer note. You want unisons or chords there. In between an occosional clash will make the last word sound better. Use this to make the most imporant points sound nice, and the things leading up to them sound worse. Your harmony can in fact change the meaning of a song.

Note that the above looked at things from the point of two parts. Adding more parts makes it tricky, one part clashing is different from the melody clashing with the harmony. If your singing with more parts you need to know what the other parts are doing, without letting them mess you up.