This is a pontentially massive topic ;-)

There's so many variables that I use to determine on how to break a
project down.  You say non-trivial.  Does that mean you're going to be
heading for an n-tier architecture?  If so I'd break each of the tiers
into seperate projects, ie UI, Business, Data Services as a minimum.
That will give you the possibility to re-use components later.

Are there going to be multiple developers working on your project?
It's important to consider how often files are going to change - the
worst thing you want is "people stepping on each other toes" and not
being able to develop if someone has a file/project checked out
exclusively via Visual Source Safe etc