Shortcut Costume Ideas
The human body has always been put together the same way, and there really are only so many shapes you can put on it. Great chunks of fashion history have been repeated in some form in the last 30 years or so, and can easily be found in thrift stores or in our own closets. Sometimes it just takes looking at pictures and saying "Wait a minute, that looks like..."
I know these are all ladies' costumes; I'll add something for the gents eventually.
Layering is the key to a lot of period looks. In this case (from the Byzantine empire), you could layer a sleeveless tunic dress over a basic long-sleeved long dress from the 70s. A lot of those wide woven or embroidered trims from the 60s and 70s are great for this stuff. The wide jewelled belt is also something you're likely to find in a thrift store. Add some flat slippers, and you're all set.
You're most likely to find something like this in a knit these days (with or without the buttons all down the front), but you'll certainly find it. The fancy tippets are simply attached to a band around the upper arm, and would be easy to make, but not really necessary. Modern dresses like this are usually mid-calf-length, but this shows how it would work to add a band of contrasting fabric at the hem. A chain belt that sits on the hips is a very medieval finishing touch, and every accessory department at the mall currently has several of them to choose from.
Layers again. This time you'd need two basic long-sleeved long dresses in contrasting colors; the bottom layer can be structured just about any way, since you don't see most of it, but long fitted sleeves would best. The top layer should be have a high waist, or be loose so you can create a high waist with a belt. Open up the front of the top layer's sleeves and hem the edges, maybe add some fancy trim at the wrists, let 'em dangle. The hair makes this look, too.
Hey, Mom, isn't this your red velvet Christmas dress from 1972? :-) This is actually layers too, but a single dress in one color with fancy trim would give close to the same impression. Again, check out the hair.
Something to do with one of those big-sleeved, big-skirted bridesmaid dresses that people dump on thrift stores in mass quantities. Find one with a straight horizontal neckline (a slight sweetheart one will work too) in a color you like, then wear a nice long-sleeved V-neck blouse under it and call it a chemise. The chain belt and jewelry (check out the chain draped across the front of the bodice but not actually around the neck) are the finishing touches. Thrift stores are full of Renaissance-looking costume jewelry from the 70s, if you never wore it or don't have any left.
Another prom-dress-with-slight-modifications possibility. This time you'd want one with a V waist, and puffed sleeves with ruffles if you can find it. (We're talking 1983ish here.) Again, add a blouse underneath (it doesn't have to be quite this off-the-shoulder or décolletée, but experiment.) Open the front of the skirt, hem the edges, and tack it back with bows (or maybe silk flowers); then add a contrasting underskirt and a hoop and/or crinolines. And don't forget the hair...fire up that curling iron! You only have to put ringlets around the edges, I promise; the rest goes in a bun.
Hey, look, it's 1968! From the neck down, at least. Another ringlet era. The bonnet is a nice touch, but only de rigueur outdoors, which we won't be. Regency hair is actually very similar to Cavalier hair, as described above, with part of it up and the rest in ringlets. Jane Austen stuff. In this era, they were even made from the same sort of small-print calicos and such that a lot of 60s and 70s dresses are.
And now we're back once more in mid-80s prom dress territory. We promise there will be plenty of room for hoops at the wedding. And don't forget your gloves!
Lots of dresses and blouses floating around from the late 70s and early 80s look like this. (Thank you, Jessica McClintock/Gunne Sax!) Some of them are polyester and frightening, but not all. I've had a lovely white Gunne Sax blouse since high school that looks like it walked straight off the set of A Room With A View.
Just a reminder that flappers don't have to wear fringe (and actually, mostly didn't). A different sort of inspiration for many an 80s prom dress, not to mention a few I saw this past spring. Gotta have the long beads, of course, and the headband with a big ostrich plume (available for a couple bucks in any costume shop) is always a fun and unmistakeable touch.