Before casting ANY spell you must first and foremost clear the area of any negative or stray energies, even if you intend on doing a curse. You certainly do not want any energies mixing with your spell work. You also should cleanse yourself of any negative energy. Before casting the spell state your intent clearly and NEVER EVER allow your intent to change during the spell as this will mess it all up. Make sure that you train your mind to be focused, do as many focusing exercises as you can prior to casting any spell. Make sure that you have a sharp clear mind that can devote all its energy to the spell. And concentrate, during the spell you must stay 100% focused on what you are doing. Failure to do any of this will result in the spell not working...or worse still, it could come back onto you.