I'm about 5'5 and 163 lbs. I'm a size 12! I know your situation.
From personal experience, I've learned a few ways to look thinner...
1. Wear clothes that actually fit well! Not too tight and not too baggy.
I used to prefer tighter clothing when I was thinner. But the clothes were so tight that my lower back (above the pant line) and a bulge, lol. Earlier in the year, I prefer baggy clothes at work. Mon- Friday, I wear heels, slacks and a simple dress top. Blah! I never wanted to catch attention in the office so I dress like an "old lady" lol. But recently, I found 3 pairs of skinny jeans (only $6 on the JcPenny clearance rack, I Heart Ronson! love it) that fit very well and make me look thinner.
2. Show off some legs! I love skinny capri jeans with wedges! My legs look longer, thinner and toned. Plus wedges are way more comfortable than heels.
3. Tanned skin and wear white! I used to tan a lot. During the summer, I would wear white (especially flowy white tops or dresses). That totally made me look thinner.
4. A belt under the bra line. It will make your boobs look larger but your mid section slimmer. I prefer thicker belts. If you live in the US, check out New York & Company. I went in 2 times this month and they have cute belts for only $9.95. Cheap and durable.
5. When it comes to tight form dresses, a body shapewear will be your best friend! Like Spankx or an off brand at any department store, Target, etc.
6. Dark jeans will make your legs looks thinner.
7. Black sometimes doesn't make you look thinner. I wear black as much as the next goth chick, but through the years, I've noticed that it's not slimming majority of the time.
8. A v-neck can make your upper body and shoulder area thinner. Showing the clavicle bone is pretty.
9. Most important! Stand straight!
When a person stands straight (even when sitting), it creates an illusion of a taller and slimmer person. Plus your boobies will look larger too. hehe
Overall, spending time trying on clothes will pay off. I can spend hours in the fitting room trying on clothes. I want to spend time to choose the correct fitting clothing. I don't want to waste money on unflatter clothing.