Almost everyone wants to look slimmer in their photos. So first rule of business is to put that chin down. Not so that it touches your chest, obviously...just a slight angle downward. And if you're a photog who knows what you're doing, you'll avoid for the most part photographing people head on or worse, from below. I try to angle my camera just slightly above so that I'm shooting downward whenever I can...it's the most flattering.
Another thing you can do is angle your body slightly to the side, not quite profile but not full on, either. I'm sure you've seen a thousand young girls on FB posing with one hand on their hip...it's because they know the secret that by accentuating that hip, it takes the focus off of problem areas and gives the illusion of a better figure.
Positioning your legs so that the one closest to the camera is slightly drawn up and bent (with your foot cocked up as well) in that half profile position will also make you look a little taller and slimmer.
Leaning against a fence with only part of your weight against it is helpful and gives you the feeling that you have some support.
Don't squish your arms to the sides of your body. I often will tell my models to put a hand out to the side to give a little motion to their photo. Don't be afraid to move a bit in your shots, dance a little...it breaks the ice and gives your arms and legs a little something to do. :) A little booty tooch never hurt anyone, either. :)
If you're self conscious and hate photos, don't hide yourself in the back. It's true that by being on the end, you're a little more exposed...but if you're in a group, get yourself between people. You don't have to be in the center, but between two people definitely helps you feel less exposed and less self conscious.
If you're posing solo, again, using the tips above will help you look your best. Also, what you wear plays a crucial role in how you'll look in your final pic. Don't wear all black! It's true that black is chic and gives the illusion of being smaller, but it's also a little on the boring side. What I recommend is wearing a darker color (or black if you must) on the largest half of your body, and then a really great color that flatters your coloring on the other half. Wearing a great scarf or large statement necklace will draw attention up to your face, which is exactly where you want it! :)
Wear clothes that fit...nothing too loose or too tight. Make sure you're comfortable in the outfits you have chosen for your photo shoot...if you feel comfy, you'll look good.
I hope these tips help! I'll do a more in depth blog in the future on what to wear for photo shoots since that's a common question I get, but for now, enjoy these little tidbits of useful info. :)