I think it was either Joe Heaney or Frank Harte (traditional Irish singers)who said that ornament should be used for drawing out a particularly beautiful part of a song. SO, look for the part of the song where the words or melody are most beautiful, and see if ornamentation will work.

Also, it gets boring to ornament the same part of a song with every verse, so look for ways to ornament different verses different, even when it's the same melody. Sometimes it's even more interesting to stop ornamenting for a line ot two, just for contrast.