E A Koetting says that if you are attached to a curse that you throw at someone, then it will mess up the curse.
It is said that once the curse, or any ritual is done, to not think on it as that will interfere with the energies.
If you invoke (from inside yourself) death energy, whether grey or black, or any of the elements and energies, then you will attach a tie between you and the other person. The emotional attachment causes that tie. You only curse yourself as you curse the other.
To evoke (draw energy from outside yourself) the energy, you keep detached from the curse and the other person. If you surround yourself with death energy (Cemetary, place of war, bloodshed has death energy), but do not take it into yourself then you will be basically immune to the energy. Then you can throw it on the person you intend to curse.
Also, it is wise to clean your own aura and chakras with a white/gold light after this to clean out any residual energy.
You might have to repeat the curse quite a few times (preferably on a waning moon. On a Tuesday or Saturday. The source is the Joy of Satan site.