Know your personal style. Interior design is all about what is pleasing and useful to you. You, the homeowner, will be the one stuck looking at the design all day and all night. So it is imperative that you know what you want.
Have a vision. Once you know what you want, form a cohesive idea of how everything is going to look together. Visualize.
Consult a professional. You may have your own style, and know what you want, but it is still best to at least consult a professional.