A suggestion I picked up from a workshop a few years back has helped me
Ask teachers for an outline of major areas they study and when. All they
need to do is look at last years lesson plans and jot down "the big
This gives you a starting point. Ask yourself what you already do that
can in some way build on the other curricula. Start looking for things
that you can use for new projects that still address the Principals and
Elements, Aesthetics, Criticism, History, etc.
I have found that it really does not matter if I teach my Aztec calendar
unit at the same time the kids are getting it in the classroom. They
either bring in their classroom learning ... or take what they learned
in art to the classroom. Fourth graders who learn about Hieroglyphs get
to 6th grade and tell their teacher what they learned 2 years ago!