I usually pinch the main color between my index finger and thumb and keep the contrasting color over my index finger so that when I lift my finger it's held with tension. Not conventional, at all, but it works for me. In fact, in the Art of Fair Isle Knitting
, Ann Feitelson does something similar, doing all the color changes with her right hand.
Most Fair Isle directions advise doing it with two hands, but I'm so decidedly right handed that I can't get a comfortable flow going with two.
It works for me.