Usually the problem is not crossing the two colors of yarn when changing colors. Bringing the new color up over the old color prevents holes and links the sections together. I would guess you've seen that twist of the yarn in the videos and in the books you've already looked at but just is case, here's a video on intarsia
and one on fair isle
from the KH site.
You could tighten up the tension just a bit for intarsia but putting more tension on fair isle just makes it pucker.