Foundation is so key to not looking "sloppy". Wear support underwear to give structure to your natural figure. Then, wear clothes that are made well and that fit well. There might be a tendency to wear sizes that fit too big, because you just want to cover it all up, but that works against you. Try on multiple sizes and find THE RIGHT FIT for that article of clothing, and don't settle for a sub-par fit. Accept that some items just will not fit you well, and leave them where you find them. You'll look better and feel better, and that will lead to you looking even better.
Because of my body type, I could always wear sizes 4-12, depending on cuts. I gained weight and started buying 16s and multiple Xs. Once I started losing a little weight, I realized that I was selling myself short with the larger sizes. I was buying for my boobs and trying to hide. I was in fear of regaining or not losing more weight, and then I realized that my clothes were too big. I look and feel so much better now. Today, I am sitting here in a 12P dress that looks really great; people have no idea that I weigh as much as I do, because of the way that my clothes fit me. I wear 3/4 sleeves instead of short sleeves--big difference.
If you have a small frame, larger Petite sizes could be for you. It sounds like an oxymoron, I know, but it could make a difference. Also, I wear varied T-shirt styles (dressed up and down) and belted shirt dresses because buying for the largest body parts can make me look waistless. I hate walking around looking like a sack of potatoes. And when I look and feel better, I tend not to sit on my butt all day.
Go to a store where someone will help to fit/style you. You might be surprised at what you walk out with. Nordstrom is good for that. Nordstrom even has a feature online that you can use to find the right style and size clothing. It's not perfect, but it is really good, at least for me.
Good luck.