Ways to slenderize your figure and make you look thinner include:

• Avoid bold patterns and prints. (When wearing patterns, use solid scarf in coordinated color to tone down the pattern next to your face.)
• Wear appropriate size prints according to your size. The larger the body, the larger the print. Avoid wide-spaced patterns with contrasting or light background as it will make you look larger. Place pattern strategically to attract eye. Tight fitting clothes with plaid or stripe design are a no-no.
• Wear your darker colors where you’re widest and lighter colors where you’re more narrow. White (or lightest value) attracts. Black (or darkest value) recedes. (Lighter colors should generally go on top near face.)
• When wearing layers, keep your outer layer in a flowing monochromatic. Add a touch of color through the use of a scarf, collar, peek a boo top under a scoop or v neck. Or try the inverse with the inner layer as monochromatic and add a complementary jacket or drapey shirt (need to leave these open so as not to break the monochromatic flow).
• Don’t cut your body in half with colors.Use less of one color on bottom and more of another on top (or vice versa) or go monochromatic.
• Use color that flatters next to face to draw the eye upward.
• Monochromatic is always a good idea as it elongates the body since there are no broken lines. Choose dark neutrals as they recede making them more slimming and absorb light making it easier to hide bumps and other figure flaws. Black, deep chocolate brown, navy, charcoal, dark grays, dark taupe.
• Shiny fabrics will make you appear heavier. Matte fabrics have a receding effect.
• When wearing oversized clothing, such as a top, choose soft, drapable, non-clingy fabrics. (Do not choose heavy, bulky fabrics as they’ll only make you look bigger.)
• Go for balance. Pair a short skirt or narrow pants with long top and/or jacket, and a longer skirt or fuller pants with cropped top/jacket.
• With strapless, off the shoulder, or spaghetti strap dresses, wear nude hose (or no hose, if appropriate). As a general rule, never wear dark hose with these as they can make you appear bottom heavy. (Always wear a slip or camisole under a clingy fabric.)
• Always stand and sit up straight and put your shoulders back. Good posture can make all the difference.
• Avoid the dreaded panty lines. Wear a thong (if you can) or seamless panties.
• Choose lighter weight fabrics. Soft, drapable fabrics will slenderize. Stiff fabrics are not good for form fitting styles.
• Horizontal stripes are generally a no-no. The eye will follow the lines around, exaggerating your width. (Exception: Thin lines that have regular narrow spacing between them lead the eye upward versus across.)
• Watch where those horizontal lines hit you. This includes the edges of fabrics. The eye will naturally follow the line around, drawing unnecessary and undesired attention to your “width”. For shirts and blouses with straight bottom, be sure the bottoms don’t hit at the widest part of your hips. You may need to tuck them in. Or wear curved bottom hems, such as shirt-tail and jersey type hems. With cropped pants, length should never hit at the widest part of the calf. Pants that are too small will create horizontal creases across the front (near the crotch area)–toss them!
• Vertical lines make you appear taller and thinner. An open jacket, drape, fold, buttons, etc can all create vertical lines. (You do not want vertical lines when they are broad and widely spaced (ex: double breasted jacket) and you also do not want vertical stripes with tight clothes as they will accentuate the bumps.)
• Diagonal lines can be used to slim if they are steep and more vertical than horizontal.
• To minimize your rearend, choose slacks and jeans with medium sized back pockets, but avoid flap pockets. Mid-rise is flattering on anyone. A midrise fit will normally hit 1 to 2 fingers width below the navel. If you have a larger tummy, avoid low-rise (and the “muffin top” look) or high-rise pants. For more slimming power, choose darker colors.
• V necks or scooped necks can elongate torso and draw attention up, especially when they are narrow and long. Necklaces and accessories such as beading around the shirt neck can as well. Wear a long necklace to add height. Short necklace can add weight. If tall and thin, a choker will “shorten” you.
• For the more maximus glutinous among us, purse or handbag should either hang under arm or hit at waist. It should never hit at the butt level.
• Make sure your clothes fit right. Do not buy clothes too big thinking they will conceal you or clothes too small thinking they will squeeze you and make you look thinner. It doesn’t work and often makes things much worse. (Pleats on pants should lay flat and there should not be any stretch lines.) Anything clingy will draw attention to your flaws and imperfections.

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As a bonus, see Essential Basic Wardrobe Pieces.
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Additional recommended reading:

It’s a Fit!: Dressing With Style, Comfort and Confidence
Color Me Beautiful
Does This Make Me Look Fat?: The Definitive Rules for Dressing Thin for Every Height, Size, and Shape