Try to make a note (on paper, on a napkin, on your hand, wherever) of great and poor designs you encounter over the course each day. From a menu to billboards to email blasts, what resonates for you? What best grabs your interest and keeps your attention and why? When you are developing your brand it will be an immediate aid to have that vision and reference for too much or too little.
No detail is insignificant: maybe you really appreciated the point size of a font used in a particular magazine ad or a commercial struck you as being obnoxious because of its background music. The more you can refine your design sensibilities by taking into account what’s already out there, the better equipped you’ll be to apply that knowledge when it comes time to make your own brand choices.