There are two small areas between the warehouses where you can stash an additional two boxes. Use these to your advantage when shifting boxes back and forth.
Find a pattern for sending boxes over and repeat it until solved. As long as you're gradually shifting the boxes to the proper warehouses, you will finish the puzzle eventually.
Don't be afraid of moving a few boxes back to the wrong warehouse on occasion. Sometimes you need to go backward to go forward.
Above all, you must find a method for moving things around.
This is by no means a difficult problem, but if you just send over boxes to the opposing warehouse, you'll block the entrance and get stuck. Think about how you can avoid obstructing the warehouses' entrances.
