I recommend wearing solid colors to help you appear taller and leaner. You could also wear a belted dress or wrap dress to accentuate your waist. Above all, avoid boxy and bulky clothing that will make you look bigger.
Layers can create a slimming effect if they are cut on long lines and start at a flattering point, like the cheekbone. Chunky layers can have the opposite effect.
Avoid wearing a t-shirt or something too casual for a top. A button-down top or a polo shirt might be good. Also, try to stick to more classic styles: nothing...