I wrote a massive post on harmonics. It's at
http://www.guitartricks.com/2000/trick.php?trick_id=5647

hope it's useful. One thing I didn't mention is this:

I used to have bad and good harmonic days. If you get pissed off with it you tend to make your contacts thicker and you mute the sound. Relax a bit. The easier you think it is the easier it becomes. Trust me!