When ordering online you should always note the Special things about you. This could be a variety of things, large arms, large legs, a small waist, large bust, etc. You'll want to ask yourself, "What body part is consistently hard to fit with my clothing." This can easily impact what size you get. Below are some tips on ordering online and fitting those "special" body parts.
Fluffy Arms - A huge one for a lot of people is the arms. Your bust/waist falls into one size category, but your arms fall in a bigger size category. I use a variety of hacks. When the jacket is lined, I take out the lining in the arms only, this usually frees up enough space. If the jacket has stretch, I get the jacket to fit my bust/waist. Finally, when the jacket is structured, I will get the jacket to fit my arms and have the bust/waist tailored to fit. I usually save tailoring for items that I feel like I'm going to keep and wear over and over. You can use these tips for button up shirts as well.
Cute & Petite - My best advice is to learn how to hem your items. When it comes to sewing, it's one of the easiest things to learn how to do. You can do by hand or buy an inexpensive sewing machine. Also, you'll want to check out the hem. Some hems aren't finished, meaning that they aren't hemmed, but just cut, this means that you cut the hem yourself to your liking. I've done this myself. When cutting a hem you want to measure from the bottom of the skirt up and mark with chalk, then cut. Don't free hand it and make sure you have a nice pair of scissors. Things that may be the perfect length for your body type are skater skirt hems and ankle pants. They are usually cut shorter.
Boota'y - I'm always having the watch for the length of my skirts. For those of us with big butts, having our skirts jacked up in the back is more annoying and could end up in an accidental flashing if we aren't careful. Most details on an item will list how long an item is. I usually like to purchase items that are 28 inches or more in length. My favorite length is 30 inches, but most places don't give you that much length. Things that are 25 or 26 inches in length are a no go for me. With dresses, I'm looking for a dress 42 or 43 inches in length. Please note that I am also, 5'9. You want to pay attention to the information that they put online. This is one of the reasons I like to order from Indie designers, they will generally be good about taking my length requests since that is an easy fix.
Boobalicious -These tips are from CeCe from PlusSizePrincess, who is more knowledgeable in this area. She says she sizes up in tops, because her boobs may make the top too short. When going for the layering look, she will wear Dickies to reduce the volume on the bust area. With a button-up shirt, she wears a tank underneath. If you have that pulling issue with your button-up then you can opt to sew the area between the two buttons that is pulling. Finally, if an item has stretch, give it a try. Items with stretch tend to work out, says CeCe. CeCe recommends Talbots and Foxcroft for button up shirts for the busty. I am going to second her Foxcroft recommendation. I have noticed the arm, and bust room is great AND the items I have gotten don't stretch. That speaks a lot for the fit.
Thighs like What - These thighs can be hard to fit. A lot of plus size clothing, especially pants, are made for apple/rectangle/inverted triangle shapes. This means a larger waist allowance and clothing that accommodates slimmer hips and thighs. For me, I will always buy that size that are going to work better for my thighs. That means sizing up one or two sizes. I am also looking for belt loops for jeans because the waist is always going to be bigger for me. High waisted items are usually a better fit as well. I can get the smallness in the waist but have room for my thighs. Stretch in jeans is always a must. I will also cut the lining out of pants for leg room.
Other things to consider when ordering online
Check the Size Chart - There is usually a size chart provided by most retailers so that you can figure out your size. You will need to know your measurements, so grab your nearest measuring tape to get you bust, waist, and hip measurement.
Size Notes - You'll also want to troubleshoot when you fall between sizes. If my hip measurement has me fall within one size category, but my waist measurement has me fall into another, if the item doesn't stretch I'll go with the larger size. Now if the item does have stretch and I will go by my waist measurement, correlate the sizes on the size chart, and then size up one size. I want to give my hips enough room, but also I don't want the waist to be super big. It's not uncommon for me to get the waists tailored down.
Know your fabrics - I wrote about this some time ago here. That will give you a more in-depth look, but the biggest thing you need to know is whether or not an item has stretch or not. When an item says 100% polyester, then it usually doesn't have any stretch, and you should use the size chart to know the exact measurements. If an item has elastane (or a word ending with "tane,") or spandex, then it will have some stretch. Stretch will give you give a little room for error with the sizing.