I don't care who you are, in a photograph that you know will be posted or shared either by you or someone else, you want to look young (or as young as possible given the realities of aging), as thin as possible, and maybe just like you stepped out of the pages of a magazine somewhere. Unfortunately you can't necessarily control when other people are taking your photo, but you can improve your own or help direct someone taking a photograph of you.
1. Looking your thinnest can be achieved by 1)actually being thin or 2)faking it as much as you can. If you are in category two, make sure before you snap a shot you are wearing clothes that aren't pulling, buttons seperating, rolling up exposing thighs, muffin tops, or belly rolls or that are so tight we can see your kidneys working. There is no amount of posing that will help poorly fitting clothing not look bad on you or you in it. If you know and loath your bat wing arms, for goodness sake, don't show up to the party where pictures are being taken in a wispy thin strapped tank top. Stand up straight. Posture will visually knock off a few pounds vs. letting it look like your slouching body is pooling in your own fat. If you hate everything below your belly button, pose in pictures behind something like a large bag, pillar, a friend, a high backed chair, bushes, or instead of posing on a bench where the photographer is in front of you, turn around a pose so the back of the bench covers up most of your body and just rest your arms on top of it and smile. If you have kids, have them sit on your lap or hold them across the body to cover up your percieved flaws. Classically you can have your photo taken from a higher angle, laying down on your stomach, or if its a head shot, push your chin out away from your neck a bit so that it helps eliminate the look of chin fat/rolls.
2. For the bathroom selfie (also works outdoors) try this little trick. Turn on your bathroom lights and stand in front of the mirror. Grab a hand mirror or sheet of foil and place it below you at a slight angle towards your face so that if you are looking in the bathroom mirror, you can clearly see the light reflecting off of the hand mirror hitting your face, and more specifically, your eyes. This should give you a bit of sparkle in your eye. These are catchlights, which even if you have dark brown eyes, will show up in your photos and make you look much more vibrant and youthful especially with a nice smile. If you're going to try this outdoors, use a large white sheet of paper or posterboard rather then the silver or glass. Also, remember a mirror reflects everything in the room with you, so try and clean off your counter tops, get rid of potential photo bombers, or crop the photo in closer. Also be aware that if you're going to go back in and photoshop your image, you better pay attention to your reflection as well because even if you're lying, it won't (unless of course you photoshop both).
3. Lighting and clarity are key. How many times have you said wow about a photo which is grainy, blurry, too bright or too dark vs. one with great lighting and that is sharp and clear? Photographers know the most magical time of day for lighting is Golden Hour. It's about an hour or two before sunset wherever you live. The light is literally golden, things sparkle, hell its down right magical! Use it to your advantage if you want a glorious glowing shot of yourself. Stand with the sun to your back so that the light creates a highlight or halo around your face/hair. Worst time of day, noon + a couple of hours. The sun is pretty high over head. If you want to know what all your photos look like during that time just straight taken outside in an open field, stand in the dark with a flashlight shining down on you. Notice the horrible shadows under your eyes, nose, and mouth. That is the noon hour. Try and avoid times when the sunlight is harsh and directly over head. If you can't, stand under a bit of shade where the sunlight is just beyond the shade in front of you to take your photo, preferrably where the ground beneath you is a light color---white, soft grey, tan, etc. As far as getting clear shots, even if you have surgeons hands, your hands still shake automatically while trying to hold them perfectly steady. To avoid this, place your camera on a sturdy surface, table, book, or if you have one, a tripod. When shooting indoors, the most beautiful light tends to be that coming in from a window in the morning or afternoon. Sit/stand in front of a window that is either in front of you or on side of you with your face slightly turned toward it. This light tends to be nice and soft and give a nice glow to the face.
4. The Photo is about you. As in the above, most successful portraits are ones where the person in them is the majority of the focus. If you posed in Time Square for example, with hundreds of people behind you, neon signs, photo bombers, trash, cars going by, etc. --that photo is about New York, not you. When you're ready to take a photo or have one taken of you, look behind and in front of you for a simple background. A nice lake, a brick wall, a solid color wall, a wall of greenery, etc. The less clutter the better. Also make sure you aren't directly in front of something like a tree or pole which may end up looking like they are hillariously growing out of your head. You also want to wear clothing that is equally simple. Big bold florals, logos, flames, or lettering and messages on the shirt/clothing distract from you. Wear solid colors, and a simple pant or jean, or dress when you're taking photos. Also keep to more natural looking make-up, and nicely shaved or groomed faces for guys.