If you have a round face and you just want to make it look slimmer, then here are some tips on how to do that:
Use make-up. Use make-up to add contour to your face. The main make-up tool to use for this purpose is a bronzer or a blush. Remember this principle: the lighter the make-up shade, the more it will emphasize and the darker it is the more it will minimize. So since you want to minimize the appearance of your round cheeks, apply a deep shade (one shade darker than your natural skin color) along the line of your cheekbones. This will add depth and contour to your cheeks. Remember to blend the color well.
Another tip is to emphasize your eyes. Here's a general rule: the bigger your eyes look, the more petite your face will appear. So put on eye shadow, eyeliner and if you feel the inclination, false lashes as well (if not, you could just opt for volumizing mascara). You'd be surprised at how slim your face will suddenly look.
Plump up your lips too; dab a shimmery gloss in the middle of your lower lip. Don't use dark lip liner, as this tends to make your lips look smaller.
Also, make sure to groom your eyebrows properly. Well-groomed and shaped eyebrows work to attract the attention towards the center of your face and away from round cheeks.
Choose a lower neckline for your clothes. One particular dress style to avoid is the turtleneck. Turtleneck sweaters and dresses will only emphasize your round face since they cover up your neck and shoulders. So stock up on V- necks, wide boat necklines, and off-shoulder dresses; in other words, dresses with necklines that don't have the effect of locking up all focus on your face. Your dresses should allow your neck and part of your shoulders to be seen to de-emphasize the roundness of your face.
Use appropriate accessories. Avoid wearing round earrings or studs; opt for long, dangling earrings to add length to your face. This willalso draw more attention towards your neck and shoulders. Coupled with your dress's lower neckline, it will make for a very glamorous effect, indeed.
You may also wear long necklaces and wide rectangular sunglasses as they have the effect of slimming the face as well.
Layer your hair. Avoid wearing your hair straight and long, as this has a tendency to look awkward on people with round faces. The best hairstyle to have is the layered look. Softly layered cuts have a tendency to frame faces so that they appear slimmer. You could also put some highlights on your hair to draw more attention towards it.
On ordinary days, you could style your hair into a high ponytail. When your hair has extra height, it draws the eye upward and works to elongate the face. It will also make your neck and shoulder area appear longer.
There you have it! These tips will effectively give your round face the slim effect that you are aiming for. Good luck!