Intent, Passion, Control
These three things are what you need most in order for your desired effect to work. Without even one of them, your spell is as effective as trying to wash the spots off a dalmation. This is one of the reasons magic can tend to be hard to perform at times. In order for you to aquire all three, I recomend for you to sit back and think long and hard about why you wish to do what it is, what you want to happen exactly (get a mental image of it engraved in your mind), and how you intend to will it to happen. Now, lets talk about the Intent.
Intent is very important. Without it your magic is capable of providing a negative effect. It is your intentions that matter most. Be it bad or good intentions. Make sure you are certain of what they are, and that you if your willing something negative, are prepared to accept the backlash that will come from it. Backlash is a neggative affect that will happen to you should you will something neggative. A backlash is something you do not want to play around with. It's much more worse than being grounded for a week, or getting fired from your job. It's something that could very well haunt you through the rest of your life.
Passion isn't something you can really think through. You either have it, or you don't. For your spell to be effective you must feel passionately that you want it to happen. Passion is the energy you are putting into your spell, your drive for it to happen. It's something you must feel deep in your heart that you need, something that you long to happen, something that you have a complete and total desire for. The more passion you have behind your spell, the more effective it is. However passioned cannot be harnised without Control.
Once you have your passion built up inside you, you need to Control it through your focus. With passion alone, your energy is going all over the place and is very chaotic. Control isn't a means to hold your passion back but rather a means of focusing it. Picture Control as a funnel. Your passion is what you pour into it, and Control is letting the passion out through it's narrow nozzel. Focusing it to a specific target or purpose.
So as you can see, ALL three are very important to wield when working with spells. Without them you might as well be a blind man firing a gun hoping to hit a bullseye.