I "storyboard" the system first, meaning I write out a design layout of how I want the system and UI's to present and flow.  Based on this, Simpl logic comes next, and this is particularly important on larger projects.  The UI is the last piece that gets done, with tweaks to both at the end.
I believe that to write a good program, you have to mentally see the code work first as your write it, then design the graphics at the end.