First, think of the desired effect you want create. Make it simple, straightforward and miraculous. Remember, most specs dont appreciate sleight of hand, they appreciate the EXPERIENCE that your magic creates. If it is confusing, or at least NOT straight forward, that will detract from the experience. Next, you should figure out all the different conditions you want from your effect. For example, if you wanted to create a new coin vanish, and you wanted to show the hands completely empty after, that would be a condition. Maybe the conditions would involve what angles it works from, if it leaves you clean, etc. Ideally, think like a spectator and decide what the effect would need to look like to convince you as a laymen, then try to build methods around those conditions. Once you have a workable method, try it out on friends and peers for feedback, then refine your method and try it out again, repeating this process til you are satisfied that your trick meets all the conditions you set out at the beginning without sacrificing the effect.
Remember, as John Born says in his book (which is where i learnt this thinking behind creating magic) "A trickster has his bag of tricks and creates moments of entertainment. A magician needs nothing and creates legends."