There are strict guidelines for wearing a Onesie. Cut out and keep by your wardrobe....
Don't go outside. If even Brad Pitt looks a bit iffy popping out to the shops in a onesie, you can rest assured it's a look best kept behind closed doors. Fundamentally the onesie is sold as a 'sleep suit', and pyjamas are for indoors only.
The only scenario allowing outside use is if you're skiing, but even then treat with caution.
DON'T cave in to the fashion police and buy a designer onesie. The romper suit's appeal lies in its cheap and cheerful anti-fashion stance. A €200 onesie flies in the face of all that it stands for.
DO research the range of onesies out there. Some are lightweight, some are practically thermal; there's fleecy, fluffy and velour. If you're short it might be wise to steer clear of onesies with feet, or risk trailing them along the floor.
(Many will disagree but ... ) DO give novelty onesies a go. Times are tough and if wearing a zebra-print romper suit with ears on the hood is going to put a smile on your face – do it! But stay indoors (see first point)
DON'T feel age is an impediment. Anyone from nine months to 90 can wear a onesie and look equally good/bad.
DO remember to wear other clothes. It's very easy to get too attached to the every-day ease of zipping up a utilitarian jumpsuit.
If you have a significant other, it's best to wear something figure-hugging once in a while, otherwise that onesie will be the only thing keeping you warm at night.