Editor's Note: That's spandex fiber. It's the safest-for-work thing I could find to put on this page about boobs and crotches.
3. Nipple slips waiting to happen
Savvy cosplayers know that you can't actually defy the law of gravity -- if some body part can slip out, it will slip out. So if you're attempting underboob, make sure you have some method of staying in your clothing that isn't just tape you bought at the drug store. Even with nipple covers, it's still embarrassing to have your entire breast fall out of your shirt, so if you insist on attempting to turn your boobs into potential slingshots, line your costume with moleskin and use medical grade adhesive. My recommendation: Graftobian's Pro Adhesive.
4. No tights
There comes a time when every cosplayer realizes that spandex secretly hates humanity and wants to make us look disgusting. The evil spandex is aided in this regard by thoroughly terrible convention center lighting, as well as seats that are made of the least comfortable materials on the planet to press against your skin. Guys and gals, if you legs are bare, no matter how great they are, Wear tights! They complete the look of even the skimpiest costumes, making them look more deliberate and less naked. Some day I'll do a column on the different types of cosplay naked. It's a topic in and of itself.
I recommend Capezio Hold & Stretch tights because they don't run, they're durable enough to stick fashion tape to them, and they last through numerous washings. The last few pairs I bought have lasted me over three years. If you don't like the line that tights create at your waist, buy the full body "body tight" style. Both of these items are available online, but any brand without a visible control top will do.
Tights of this kind prevent camel toe, wedgies, and wardrobe malfunctions, hide goosebumps in overly air-conditioned buildings, and maintain the look of your legs later in the day when they get tired. They also prevent blisters on shoes you'd otherwise wear barefoot, and hide if your bikini wax isn't entirely up-to-date. If your costume calls for fishnets, wear a pair of these tights underneath so the fishnets go on smoother. (Note: the aforementioned metal bikini is one of the rare exceptions to the "always wear tights" rule, since there is no practical way you can hide the waistband in that costume.)
5. No dance belt
This is a special note for male cosplayers: always wear a dance belt! What's a dance belt? It's like a heavy duty jockstrap that doesn't leave lines. No, you cannot just re-purpose the jock strap you use for baseball. It will show under spandex and make you look more like you're into bondage than superheroes already do.
A dance belt serves both an aesthetic and a practical purpose. What's that practical purpose you ask? Injury prevention! I'm not just talking pinching or squashing. If you plan to be around kids, an athletic cup may even be in order, because kids seem to have the accuracy of marksmen when it comes to hurtling their head, shoulders and elbows directly into the groins of guys in superhero costumes.
If you absolutely do not want to wear a dance belt then spandex costumes are not for you. Choose something significantly less clingy. Otherwise, expect to get the dance belt talk from that super cute cosplayer you are freaking out with your dangly man bits. The dance belt talk is very real, and it's really awkward for everyone involved. Consider this your dance belt talk. Internet anonymity can sometimes be a good thing.