I find it is easy if the melody follows the progression you are playing and it only gets difficult if you are playing a break (or something that requires a lot of concentration) and trying to sing / doing backing at the same time. However, as ever Noticingthemistake has it spot on, practice is the answer !
My
instructors page and
www.studiotrax.net for all things recording.
my toons Brought to you by Dr BadGAS