I'd build up the area to promote drainage, and incorporate a lot of stone and grit - pea gravel, pumice, turface, et cetera.

Josh

What Josh said, notice he didn't say "sand". In order for sand to help clay in good old earth takes a HUGE amount of sand. If you add too little you will make cement.

tj

Wow i could see myself accidentally making cement

Build a raised bed and fill with an appropriate soil. Don't amend the clay soil under the bed. You'll need a way to use gravity to ensure water drains away from the site, so a trench, shallow ditch, or tile to ensure the 'bathtub effect' that will happen under the bed w/o a drain (even if you don't amend the native soil), won't/can't occur.

Al