Your intentions have alot to with the course actions of your spell. Myself, i have a simple code i go by to insure my spell do as directed, ,mind you, i create my spells like there actually alive:
First, always let the words of spell come to me, so if i am sitting in a Diner and a perfect set of words comes across my mind i write it down.
Then i decide how to make the body of the spell, filling it with candles, incense, oils, and other substances. Each item in its place, each one falling on the property's of my spell.
After, i have the foundation, before i cast i give the spell purpose, a reason to exist. When i find its a reason to exist, i then give it a deadline, how long will i keep this spell going?
When, all is said and done you give your spell the touch of energy to give it life.
Hope this could help,