Once you have sold a vision and made the commitment on behalf of yourself (or your staff) to actually create a product, the magic of conception unfolds. Somewhere deep in your conscience you have the basic concepts of how the final product will look and behave -- in other words you've got the general parameters of a target -- but how will you get there?
Rather amazingly in our modern gadget-driven world, the best approach is still to sit quietly in a nice sunny room with a window, a pencil, and a pad of paper, and begin sketching. Draw out your concept, write down your concerns, and show all of the pieces and how they relate to one another. Make lists of the things that you need to store.
Then you're ready for some more presentable modeling. I usually like to start with three diagrams: a state model, a sequence diagram, and either an entity-relationship diagram (if you're focusing more on the data structure) or a UML diagram (if you're focusing more on the objects). The purpose of all this however isn't actually to convince anybody of anything, but rather to help clarify your own thinking and to pique new insights.
Now put the diagrams down and go out for a walk. Or better yet do the diagrams on a Friday and spend the weekend exploring around town.
On Monday look at your drawings with fresh eyes. What did you overlook? Things always turn out to be more complicated than you initially anticipated. Who do you need to talk to in order to clarify matters? Are there more robust ways to create your solution? Something that might be more flexible?
This first couple weeks of a project are magical but they are also a bit frightening. We make architectural decisions that fairly rapidly get set in concrete. So don't rush yourself through this aspect of design -- consider all of the possibilities. Then after you've nailed your design parameters comes the real stickler, user interface design.