I have beautiful Asters that get very leggy and too often do not resemble the beauties I originally bought . Can someone explain the proper technique for pinching?

if your annuals have leafy stems, cut or pinch off the spent blooms or bloom stalks just above the top set of leaves. cut leafless stems back to a main shoot or to the base of the plant. if you plan to save seeds, stop dead-heading by late summer to allow some seeds to form.
hope this helps
sibyl