Fashion:Find underwear you feel pretty in.
And that fits and are practical enough that you can wear them on regular days. Good underwear/bra combos are the basis for everything. If you know you look good without your clothes on, you'll look better while you're wearing them, too. Save a few of the ugly, plain cotton ones for days when you don't want to ruin the good ones, and toss the rest. If putting it on doesn't make you feel good, there's no point.
Buy a short satin or silk robe.
You have a robe, probably. It's probably terry or fleece and warm but supremely unflattering. Get a satin one and wear it around the house during "lounge" time. It's no more effort than the ugly one, and makes a good deal of difference. Leave soft blankets around the couch or your favorite chair to curl up in if you get cold.
Get some cute things to sleep in.
I don't mean flyaway babydolls or corsets. They have their place, but sleeping isn't it. Boxers and a tshirt are comfy, but so are tap pants and a cami, and those are infinitely prettier. Short nightgowns are also a good plan, and again, take very little effort.
Upgrade from your sneakers.
Sneakers, as European women know (and Americans seem to ignore), make everything else you're wearing look sloppy. Get some pretty ballet flats, and a pair or two of heels you can actually walk in (nude and black pairs are most versatile). Instead of cheap, boring flipflops, get pretty sandals instead (thin leather strapped ones are so much better). Save the sneakers for working out.
On a related note: learn to walk in heels.
This means, first, buying heels you can walk in. When you stand straight in a pair of heels, the weight of your foot should be on your heel, NOT on your toes or ball of your foot. If it is, don't buy them, no matter how cute they are, because you won't be able to wear them for more than a few minutes without pain.
When you walk, put your heel down first, then your toe. Make sure your shoulders are rolled back and down.
The accessories make the outfit.
Really. The difference between looking polished and like you just picked up what wasn't already in the dirty laundry basket is the accessories. You have jewelry--wear all of it, not just the same stuff you always put on. Get a few large/unusual pieces, too. Get a few scarves.
Start reading street fashion blogs.
I recommend The Sartorialist, Garance Doré, and Style Bubble. They're mostly pictures, so they're not all that time consuming. Street fashion is better than looking at magazines, because there's authenticity. Magazines are looking for a good image; real people are looking for a good 3-D presentation, and they tend to be much more creative. Learn to be creative, too.
Make-up-y things:Get smooth.
Being feminine means, among other things, being soft. Find a lotion that works for you (it doesn't have to be expensive, just effective. Lubriderm or Curel for sensitive skin work well for me). As soon as you get out of the shower, and after only the barest amount of drying off, use lotion on everything. If you dry off completely, or wait until your skin is totally parched, the lotion won't be as effective and you'll have to use a lot more of it.
Sugar or salt scrubs every so often while in the shower--and a pumice stone for your feet--are good means to this end, too.
Additionally, you should be washing your face, and you need to moisturize immediately after every time you wash. It'll keep you from breaking out.
Invest in a good haircut.
You wear your hair every day. If you get a good, flattering cut, you won't have to spend as much time on it, and you'll automatically look better. This is worth paying for (I spend $60 per cut on mine, but then I only have to do it every 6 months or so). Straight, mostly blunt cuts from CostCutters just don't do much for anyone.
They matter. If your eyebrows are done, even a totally bare face seems put together. If you can't figure out how to do them well yourself, or can't be bothered to maintain them reliably, find a salon you like and get them waxed. Ten bucks a month is not a huge investment for such a good payoff.
The cosmetic, not the verb. For some reason, it's one of the most over-looked cosmetics, and it does such good things. If you're having trouble finding a flattering shade, I'd recommend L'Oreal's Super Blendable blush. Use the foundation sheet to figure out your skin shade, and then get the blush that matches. It's hard to overdo it on the pigment, since it's not as saturated as other formulas.
Miscellaneous:Buy yourself some flowers.
Boys are dumb. They rarely think of things like flowers, and if they do, it's only because it's some sort of occasion (Valentine's day, making up after a fight). Fresh flowers are too awesome to doom to such miserable connotations; buy them for yourself instead. Trader Joe's usually has a huge selection, and they're often quite cheap. Put them wherever you're going to see them often.
If you do everything I listed above, your life will be better, men and women will fall over themselves to get you, you will be happy and fulfilled and supremely feminine at all times, and nothing will ever go wrong ever again and you'll live happily ever after.*
*That's how the magazines always end these sorts of articles, right?