This page is intended to help advise those who are looking to design superhero costumes for a "Silver Age" sort of setting (like Crucible City MUX), or "Pewter Age" (like C2MUX) where heroes are generally heroic and larger than life. This is not for "realistic" settings, or for designing costumes that you, personally, might look good in. In fact, that brings us to Point One:
There are a number of things you'd want to have in a costume that you intended to wear in real life, many of them intended to hide the fact that you don't have a chiseled physique (if you do have a chiseled physique, congrats). Things like leather jackets or trenchcoats are very popular here, but with only a few exceptions, these don't fit the Silver Age look. This point is weakened a little foor Pewter Age stuff, but should still be kept in mind. Coats are very 90s, and that fad has largely faded. That ties in to Point Two:
This goes beyond the fact that Liefeld is a bad artist. While he was briefly uber popular, his favored design motifs got extremely overused. So, while not all of these go against the Silver or Pewter Age moods, you should probably avoid too many of these elements in your design. Liefeldisms include, but are not limited to:
As a rule of thumb, more than three distinct colors (as opposed to hues of the same general colors) on a costume runs the risk of tackiness. When you hit six, you'd better be shooting for either a rainbow theme, or an attempt to emulate Captain Ultra. Can't draw? At least do a rough crayon drawing of your intended color scheme to see if it's too busy.
Spider-Man can be perfectly sneaky in bright red and blue. Batman spent decades in mauve and blue with a yellow oval on his chest. You don't need to wear all black to be stealthy...this is a comicbook setting. Unless you actively GLOW, no costume hinders attempts to hide. Again, don't get hung up on realism! Also, this isn't World of Gothness, you don't need to look all desolate and bleak. If your outfit is more than half black, reconsider the design. If your outfit is more than ninety percent black, burn it and start over. If your outfit is entirely black leather with silver accents, you probably need to rethink your motivation for making a superhero character in the first place. You're not a Splinter Cell infiltrator or a cheesy ninja (and even if you're a cheesy ninja, you can wear other colors). Broaden your range of color! At the very least, consider grays or dark blues and dark greens.
The Incredibles aside, capes can work just fine (just make sure they're not too tightly attached!). But they shouldn't touch the ground when you're standing straight up, unless they're actually part of a hooded cloak. Cloaks are okay if you're a mystic type or a dis-timed old time sword-swinging anachronism, but should otherwise be avoided. Capes longer than this are going to draw unfavorable comparisons to Spawn and mid-90s Batman ("Shall I fetch the posing cape, Master Bruce?"). Full capes should at least drop to mid-thigh, while half capes (a la the Marvel Family) should be no longer than this. Short capes that don't even reach the belt line work for female heroes, not so well for guys.
Okay, here's one place where I strongly advise going against the classic designs, because they're just plain stupid (especially these days). While women are encouraged to bare their legs, men should keep their legs covered unless they're doing the Tarzan look or the Namor-style "swim trunks and an attitude" costume. As a rule of thumb, if your costume includes covered arms and/or hands, the legs need to be covered. Otherwise, you look like you're cruising for a good time at a gay bar. And even if your character is gay, it's not really a good look for a regular costume (save it for off-duty time). I suppose a kilt is okay if your character is Scottish, or a toga if you've got a Roman/Ancient Greek thing going (although you might want to wear it over a bodysuit in Crucible City...it gets windy).
Unless your schtick is being an Armored Hero, you probably shouldn't look visibly armored and padded. Silver Age heroes were all about kinetic weaves and microfine chainmail and other protective clothing that didn't look obviously like armor, and Pewter Age tends to follow that for the most part. And while helmets, elbowpads and kneepads are recommended if you go out skating, they can look awkward unless you're already bulky and/or want to evoke a football uniform. So even if you took Armor for your character, it doesn't have to look like a flak jacket (so long as there's some clear sign that it's protective). Unless, um, it is a flak jacket bought with Equipment, of course.
I may add to this as I think of things or get suggestions I like.