Two chiffon blouses: The one underneath should be slightly snug (to avoid bulk) and in a paler hue. Top it with a darker blouse that has a deep neckline so it works almost as a super-light jacket.
A boxy top over a tank: To get a retro-cool effect, wear a cropped, roomy blouse over a fitted tank. The tank will define the waist and create a lean silhouette so you can go for something really fun on top.
A knit tank over a long-sleeved tee: For a French spin on layering, wear a swingy knit tank over a long-sleeved, fitted tee. Keep the two items in the same color family for a more cohesive look.