The way you dress can make a big difference in the way you look. Not only can you appear more professional or more relaxed depending on what you wear, but you can also look thinner depending on your choice of wardrobe. If doesn't matter what your dress size is, or whether you are bottom-heavy or simply out of proportion, by adjusting your attire you can make a big impact on your looks.
Probably the number one mistake people make is to wear clothes that don't fit. Wearing larger sizes does not make you look thinner. In fact, it will make you look even larger. On the same basis, wearing a size 12 if you are a 14 will not make look like a 12. All it will do is to accentuate every single area in which your body is hiding extra fat. Wearing clothes that fit may seem like an obvious answer, but if you want to appear thinner, this should be the first thing you do. The same holds true for fabric. Avoid clingy outfits (Lycra and silk are difficult to wear properly) and instead choose pre-shaped outfits that will give you a general outline and make you look thinner.
If you need to look smaller on the bottom, avoid pleated skirts (they make your stomach look bigger) and instead choose A-lineskirts. If you need to hide bulges, choose loose-fit and low-ride jeans. With low-ride jeans, however, make sure to wear a top that is long enough. Boot-cut pants will balance large hips, and simple designs without a lot of extra pockets will also make you appear thinner. Avoid slim-fit jeans if you have large thighs, and opt for darker shades to minimize trouble spots.
If you are trying to make your upper body look thinner, avoid tank tops in summer, as they emphasize your arms. On the other hand, if your arms are toned, showcasing them will take the focus away from your problem areas. Boatneck tops are great for balancing your body, and something you should definitively use if you have good-defined shoulders.
Finally, good posture is key to looking thinner. Throw your shoulders back and do not hunch. Keep your head up high and look confident. Wear heels if at all possible, especially with jeans, as they elongate your figure and make you look thinner overall.