I shop online all the time. I actually do like 50% of my shopping online. It's easy, it's fast, and there's a much greater selection than there is at local stores. I thought online shopping was a totally natural thing to do for shopaholics like me, but I was really surprised to hear that a lot of people I know flat out refuse to shop online because they can't try things on, they think they have a weird body type and nothing fits them (Maud), they can't see the clothes in person, etc, etc. And I just have to say that all of that is silly. Online shopping is easy and can work for everyone. And it's addictive, but that's another story.
For those who want to be able to see clothes in person, I have to say they really do generally look like they do online. If you're really unsure, look at the thing you want to buy at a bunch of different sites. If the color/fabric finish differs drastically on each site, email a customer service rep and just ask them what it looks like in person, if the blue dress is more of a green blue or a true blue, if the denim is a thick or a thin weave, you get the drift. Customer service is there to help you. That's what they get paid for. You can also ask customer service to measure something for you, so you can compare the measurements to your body and know you're buying the right size.
As for things fitting and looking flattering, you really have to know your body type. I'm just going to use myself as an example because, well, I'm right here, and I'm lazy. I sometimes have trouble fitting into dresses because I have really hourglassy figure, so my chest and hips/thighs are almost a size six, my waist is a small four, and my legs are on the thin size. So stuff that fits me in the chest doesn't fit me in the waist, and stuff that fits me in the waist won't zip over my chest or my hips. Not fun. Sometimes I'm a 2, sometimes I'm a 4, sometimes I'm a 6, depending on the cut of the dress. Now, I know it sounds like I really shouldn't be shopping online if I'm not a consistent size, but I have a strategy. First, look at the cut of the dress/skirt/coat/etc. If its a trapeze shape, AKA with very a-line shape and no waist or chest definition, I'll order a 4 because I don't have to worry about my hips fitting into the roomy shape. If it's fitted around the bust and tight over the hips, I might order up a size. I know what you're thinking. Ordering up a size means it won't fit what parts of you are the next size down. That's where my next tip comes in.
Get it tailored. I can't say this enough (if you watch What Not to Wear, you know where I'm coming from). Tailors are like the fairy godmothers of online shopping. So when I buy that dress one size too big, I can take it to my tailor and have it taken in at the waist, shortened, etc. That means if you're unsure about if the perfect dress you found online will fit, you can make it fit! Never, ever buy something in the smaller size, however. Clothes used to be made with lots of fabric left over in the seams (AKA seam allowance) for easy alterations, but today designers have gotten cheap and you'll be lucky to have a 1/4 of an inch at the seam. 99.99% of the time you won't be able to make something bigger where it doesn't fit you. 95% of the time you'll be able to make something smaller where it's too big. Buy big, not small.
Not everything is worth getting tailored, however. If your shopping on the Forever 21 website, or the Old Navy website, or somewhere that sells cheap, trendy clothing, it's not worth it. You'll spend more money on tailoring than on the clothing itself, and you'll only wear it for a season. Don't waste your money. Also, be aware of what can get tailored and what can't. Denim is easier to alter than pleated chiffon or intricate beading. Don't buy something you're not sure can be tailored. Some alterations, like hemming jeans, you can actually do yourself pretty easily, and save a bunch of money.
I have lots of examples of getting stuff tailored that I bought online, but the most recent is my graduation dress. I fell in love with this Abaete dress last year (the belt's not included). But the fit across the chest is so essential to the look of the entire dress, and I was worried about the four being too small. So when I saw it on eBay (AKA the best online shopping site EVER) in a 6 for a seriously reduced price, I snatched it up. And I love it. All I have to do is get the shoulder strap tightened and the waist taken in a bit, and it is perfection. Love love love.
PS: Don't be afraid to shop on eBay. I know, everyone always gets suspicious when designer clothes are being sold for way below retail, but if you do a little research and check out the seller's satisfaction rating, you can usually avoid being sold fake goods. I've bought probably 100+ things on eBay, and only one has been a knockoff (and I got my money back. eBay's really great about that). I got my grad dress $300 off, and I just got my prom dress $200 off. A lot of times people buy something, don't like it, lose the receipt, and can't return it. Or wear something once and decide they don't want it anymore. Or a few sequins fall off a dress and someone would rather get rid of it than fix it. That's where eBay comes in. It's a great way to take advantage of recession deals!
Abaete Dress: style.com