I'm always scared to order things online because the sizes in my own closet vary so much.
One thing that's helped is going into multiple stores and just trying stuff on to get a vague idea of what my size is. I tried on some things at AT Loft, and the Gap in the past few weeks, and now I have a better idea of what sizes I would wear in those stores, and their 'sister stores', at least, I hope.
I learned the other day at Talbot's that I wear a certain size in a straight skirt there, but need 2 sizes smaller in an A-line skirt. That's good to know, and I'll remember that when I order from other stores.
I look at online photos more for color and detail than fit since I'm probably 6 inches shorter and 30 pounds heavier than most of the models shown.
For me, as a pear, hip measurement is usually the deciding factor, so if the given measurements are correct, (and I'm not lying to myself that day), I can sometimes order the correct size. Go figure.
I'm reluctant to order from stores with expensive shipping or that don't have brick and mortar stores I can return items to. Period.
Oooh, but wouldn't it be nice if online stores showed different women wearing the same items?! As in: "Here's the regular model, here's a short pear shaped woman wearing the same dress, and a tall rectangle, and an apple..."