Smooth Strokes Without Direction
Practically invisible strokes are those that seem to have no beginning or end. They overlap in swirling directions giving the illusion of smoothness. This can be accomplished by remembering to slow down and go dark gradually, and at the same time varying the direction of the strokes so they blend together smoothly. Inconsistency is a real plus for achieving smoothness, strange as it seems.
Stokes that all go in only one direction are also an effective way to fill a shape. The beginning and end of each stroke can be a bit tricky. The goal is to see the strokes without seeing ragged ends, very much like having a good haircut.