It's all about that little groove in the ankle. The top edges of ballet flats often follow one even line, running right across your ankle bone, which can rub them raw in this very miserable way. Not these. They're genius.
Try a d'Orsay flat. Instead of a snug top section that cuts into the middle of the foot, they're intentionally loose, so they wear sort of like a slide sandal. (To me, they feel like wearing Adidas shower sandals. So easy and don't-even-think-twice-about-them.)
d'orsay flats in caramel, $495, jenni kayne, shopbop.com
Look for a slightly raised heel. If you're going to be standing for hours, you don't want to be completely flush with the floor. It's much easier on your back to have the slightest, teensiest bit of a heel.
Moccasins + rubber soles: it's an unbeatable combo. The top's super-soft, like you're barefoot, but the sole's got enough insulation and structure so you're not flopping around. They're the best non-shoe shoe.
"el paso" II moc shoes, $108.61, minnetonka, asos.com
Pick a Mary Jane flat. Sometimes flats are ever-so-slightly too big and I sort of have to fight to keep my feet in them, which strains the muscles in the top of my feet. It sucks, and it's why a good top-of-the-foot strap is key. (Not an ankle strap, though, 'cause those tend to be killer.)
Maria Sharapova is the patron saint of women with blistered feet. Everything she's designed with Cole Haan is genius. These have the best elastic and adjustable strap, so you can secure them on your feet in the exact perfect way, not a millimeter too tight or too loose.
air "bacara" ballet flat, $158, maria sharapova for cole haan, colehaan.com
Make sure your rubber flats are on the thinner side. If they're made of thick jelly material, they won't have the flex of thinner rubbery shoes like these, which move with your feet instead of against them.
Find pretty espadrilles. Espadrilles are almost always ridiculously comfortable, but they're not always fun to look at. But not these! They don't look like fisherman shoes; they look like fancy shoes you can wear with eyelet dresses for all your summer dinner parties.
Know your feet. Narrow feet need narrow shoes or they'll keep sliding around in all the wrong (blister-giving) ways. Don't assume you don't have narrow feet just because you've never checked. (Or, for that matter, wide feet.) In either case, the right width makes all the difference in whether flats are comfy or not.