Make your costume CONFIDENTLY. Nothing's sexier than confidence.
The more skin, the better. That's why snakes are so sexy - they have so much skin THEY'RE CONSTANTLY LEAVING SKIN BEHIND. Basically, make your costume a confident snake.
Know how you do sex? With genitals. Know what has double the genitals of a normal person? The platypus. The male platypus has a double-headed penis - basically, it's got TWO dicks. Talk about sexy!
And there's no animal as in touch with their sexuality as bonobos. Bonobos are the only animal known to have casual sexual encounters for pleasure instead of to produce offspring (other than humans). They engage in oral sex, tongue kissing, and more. What makes them even more sexual than humans though is their significantly laxer social customs - it's not uncommon for bonobos to engage in sexual activity with members of both sexes frequently, and they rarely (if ever) form permanent monogamous relationships - meaning they're swinging to different sex partners ALL THE TIME.
Know what virtually every poll ever taken will rank as one of the sexiest traits for someone to have? Having a sense of humor. And who has a better sense of humor than Hippocrates, inventor of the humoral theory? Cover yourself head-to-toe in blood, yellow bile, black bile, and phlegm to show potential hook-ups you got ALL the humors.
Time to show off your SEXY COSTUME - a confident snake-bonobo-platypus hybrid covered in various bodily fluids!