Plan out your spell very carefully. Get a piece of paper and a pen or pencil and map out what you want the spell to be for and materials(crystals,herbs,salt)/ritual performance(chanting, dancing, etc.) you can do to help or enhance the spell.
Another thing is that I find helpful is meditating before creating the spell. Your mind will be much clear and perhaps have more energies flowing around and maybe a good idea for your spell might just come along during or after the meditation.