Straight motions, i.e. back/forth, up/down, are almost always best. Some manufacturers suggest circular motions to ensure complete coverage of the product. I believe this is the case for most waxes and sealants. Abrasives however, may benefit from a circular motion becuase of the heat generated and the quicker action to break down into a finer polish. As far as applying a common wax or sealant, I always use straight...
:wavey