A good tip for any loose weave fabric that likes to stretch across bias cuts is to stay stitch the pieces. This means, stitch around all the pieces flat near the edge with a fairly tight stitch. This will help prevent areas like the armscyes and sides from stretching and warping while you're working with the pieces.
Also, before you lay in a lining or a hem, hang the completed shell on (preferably) a form or a moulded hanger for a day or so to let the weave drop and settle. You can then even the hem and will avoid buckling and sagging later on.