Watch the Crosspicking online guitar lesson by Roberto Dalla Vecchia from Solo Flatpicking Handbook
Crosspicking is another technique that comes handy when we want to fill up holes between melody notes. The most common pattern is 4 beats long and can be played on any three adjacent strings. This pattern creates a very interesting tension in the rhythm and it works great against a bass/strum back up.
I usually prefer to play crosspicking using two downstrokes in a row followed by an upstroke, but it's possible to play it with alternating strokes as well.
Crosspicking is very demanding for the right hand, so be patient and build up your speed slowly. When speeding up, make sure all three notes have the same volume.