I swear by tamiya tape. This stuff is far suprerior -edit-
to-edit- blue or green painters tape in every aspect i can think of. Although i might have to try bare metal foil some time.
If you are applying thick layers that could also be your problem, even if it results in the same amount of paint in the end. Thinner layers will give a flat even coat and not build up along any edges.
Also as was recomended, try spraying at a 90 degree angle.
Mongol General: Conan, What is best in life?
Conan: To crush your enemies, see them driven befor you, and hear the lamentations of the woman!