First of all, be proud of your best features and show off your assets. If you have great legs, wear a skirt and heels. If you have the most beautiful clavicles known to man, wear a shirt that shows them off. You can always stand to get more attention for what you've already got going for you.
Next, base your wardrobe on darker colors. Darker colors won't make you seem like you're bigger than you are, which is a nice thing when you're already happy with the size you are or wish to get smaller. You can always try wearing a monochromatic outfit, which can make you look taller, too. Also, to have longer, leaner lines to your body, you should only wear vertical stripes. And for your own sake, don't go to the store and try to fit into a size. It's only a number and you need to have clothing that fits you, not the other way around.
You have the option of sculpting undergarments, but you'll have to remember to wear them all the time if you only have clothes that fit you when you are all pushed in. Keeping in line with that train of thought, don't wear anything that is too tight or clingy. You will look uncared-for and it will be unflattering for your full figure. You need some tailored clothing to set your curves off right. Tailoring can be surprisingly inexpensive, so take a pair of pants that fits, but could do with a little shaping, and find someone who can do a good, professional job of tailoring them. You'll be amazed at the result.
Don't choose large swaths of fabric covered in small prints. They will only make you look disproportionately large. Instead, choose solid colors or, if you like prints, prints scaled to your body that overlap, as empty space between components of a pattern will also make you seem overly large.
Most of all, love your body. It's one of the best gifts you'll ever get. Work with—not against—it and you will find that you will be much happier and feel a lot more peace when you walk into the clothing store.