For the gents
Stop wearing t-shirts under your open-collar shirts. This is my #1 tip for looking more like a grownup and less like a d-bag. Are you ashamed of the little notch of skin at the base of your neck? The whole point of a collared shirt without a tie is to show a bit of skin, which looks relaxed and therefore sexy. By covering that skin up with a t-shirt, you're basically saying, "No thanks, I don't care for sex." Sometimes the t-shirt thing can look OK in a hipster kind of way, like if it's a bright red tee under a western shirt or something, but even then it would probably look better without.
Don't wear socks with shorts. I think shorts can look pretty sharp on a man if you have a reasonable physique, but for Christ's sake don't ruin it by wearing them with socks and sneaks. If it's hot enough for shorts, it's hot enough for flip-flops (or some sort of slip-on).
When it comes to layering shirts under sweaters: A shawl-collar sweater looks infinitely better with a dress shirt than a crewneck. A crewneck sweater on the other hand looks better with a crewneck underneath. Like likes like, except with a v-neck sweater you can go either way.
For the ladies
Try wearing a shirt over a maxi dress. The point is not to make it look like you got a new skirt, so it works best if the shirt and the dress have different necklines. For example you could wear a scoopneck top over a halter dress or a tank top over a racerback dress, etc. This is one of my favorite tricks lately, especially for work and travel because a) long skirts are hell of comfortable, b) it looks summery but is not too cold in air conditioned conditions and c) is a little more covered up around the torso, so I don't have to worry about slouching wrong and giving someone a sideshow.
Buy an LBD (little black dress) in a jersey fabric, preferably with some kind of embellishment like ruffles or an asymmetrical neckline. It will be comfortable and easy to throw on when you don't feel like trying too hard, but looks fancier than it is, and can be dressed up or down easily. It's also all-season: wear it with tights and a cardigan in winter, with sandals and a denim jacket in summer.
Buy some cropped pants. They take 10 pounds off (or something, I don't know, this isn't an exact science like gluten-free baking). Cropped pants make you look skinnier because they show off the thinnest part of your leg. If you hate your ankles, obviously, ignore this one.
If you're not all that adventurous, just buy distinctive, colorful accessories (glasses, hats, scarves, shoes), wear them with your regular clothes and everyone will think you are adorable.
Colors can be neutrals. Which is to say, don't automatically buy everything in black or brown because you think it goes with everything. Deep red or navy goes with almost everything, especially if everything else you own isn't brown or black. Charcoal gray and olive green are other good neutrals. I'm thinking especially of coats, shoes and bags but also belts and even pants which you want to be able to wear with lots of different shirts.
Don't buy the same item in multiple colors/patterns. This may go against popular opinion, but I maintain: a) You'll end up loving one more anyway, and b) you're not fooling anyone. Instead figure out what you like about the thing (the cut? the fabric?) and look for similar but not identical items.
Do my stylish readers have any additional tips/brilliant ideas to share?