Berkeley has a pretty good guide for writing a Statement of Purpose:

http://career.berkeley.edu/Grad/GradStatement.stm

It's more for "academic" rather than "design" grad programs, but still gives a pretty good outline of things admissions committees are looking for in a statement.

I've written a lot of these over the past twelve years (fourteen, if my count is right, and that doesn't include the four that I just finished for Ph.D. program applications). The only specific advice I can suggest is to have several versions, one for each program to which you're applying. Make sure the admissions committee can tell you know something about their program. Don't be afraid to mention specific faculty members, courses, overseas study opportunities, etc. In your mind, you should be able to answer the question "Why are applying to OUR school?"

Hope this helps.

thanks for the help snjr