As mentioned, look for a shop that stocks helmets and go try one on. If anything, for the correct size. Also possible, go to an autocross with a new headsock and try other people's helmets.
- Find one with removable liner. You will sweat. That sweat will go somewhere. Headsocks help, but that sweat eventually gets into the liner. Three years down the road and you're gonna start hating putting on a sweaty, old helmet.
- Find a helmet built for HNRS (Head, Neck, Restraint Sytem), aka "HANS". This will save you from having to drill the helmet later to adapt one if you ever want to do Lemons, or W2W, or just have more safety.
- Full face vs open face is more of a personal choice. I personally like where my jaw is, having a visor, and the added fire protection that a full face helmet provides. I've had pebbles get kicked up and tossed thru a FRS's open window and smack me in the helmet while on track. I also don't like looking like I'm cosplaying as a Spaceball character. Additionally, I've only see open face helmets used abundantly at autocrosses. So, consider the motorsport activity when choosing a helmet.
- Removable cheek pads are nice if you plan on wiring in comms or want to shave them down for a better fitment.
- Carbon fiber helmets are nice, but the real weight savings isn't really seen until you start spending +$1000. So, if you are eyeballing that fancy CF helmet, but it's $200 more, just save that $200 on a track day. CF lids are very helpful in cars that generate a lot of lateral Gs (ie. aero heavy cars) and where you are doing enduros.
- For fitment, if you shake your head with the helmet on, and it moves more than your head, it's too big. If your cheeks are pretty red after wearing for 10 minutes, it's too small.
- Once purchased, wear the helmet for an hour around your house. It'll help break it in.
I've personally used Pyrotech, Bell, and Arai. The Pyrotech felt like a Budget rental car. Bell feel really nice and have a decent size eyeport, but they lack removable liners. Arais feel great, but can be pricey once you go up the feature list.