Nina Zanetti’s 12 episode lesson set on playing expressively has all kinds of nuggets in it for musicians of every kind.
Her specific instructions and suggestions apply to the mountain dulcimer, but are applicable if one is a singer, or hammered dulcimer or harmonica player.
Check out the introduction to the series:
The rest of the series is available at http://dulcimercrossing.com/md_ex/exprs/dc_exprs_pre.html. Just sign up and you can see them all!