Choosing the right full face motorcycle helmet is easy when you know what to look for. I'm going to go through the steps and list what it takes to get the right full face helmet to protect you. As I always say, dress for the crash and not the ride.
First off you need to get the right size full face helmet
Most manufacturers helmet sizes vary so much even if a Shoei small full face motorcycle helmet fits you perfectly a small Nolan full face helmet may be too small. Always try them on, after all, a helmet that doesn't fit isn't going to protect you correctly and may even come off during an impact.
Put the full face motorcycle helmet on. It should be snug but not painfully so. Just like a pair of shoes, if it's tight in the store it'll be unbearable after you're on the road for an hour. There should be no uncomfortable pressure anywhere, especially the ears. If you move the full face motorcycle helmet left to right and up and down your skin should move with it. You should also not be able to pull off the full face motorcycle helmet from the back lifting towards the front.
Should the helmet be DOT approved?
After you get the right size you should make sure the full face motorcycle helmet is at least DOT approved. Even better is a SNELL safety rating. If the full face motorcycle helmet you want does not have at least a DOT rating it just isn't safe, and is probably not legal to wear in your state. If you're shopping at an overseas site you will probably never find a DOT approved helmet since this is an American specification. If your state requires a DOT sticker on your helmet avoid overseas sites unless they can give you documentation that the helmet is indeed DOT approved. Keep in mind even with the documentation in hand you'll still be pulled over more often than you think for not having the sticker on the back of your helmet. In a state with no helmet law you won't have to deal with this.
Make sure there is a secure strap with a D ring. Secure the strap and once again make sure you cannot pull of the full face motorcycle helmet at all. A helmet that'll fly off your head does little good in a crash.
Next make sure the full face helmet has vents. Not only does it make it more comfortable in the summer but will also help prevent fogging.
Next up choose the color. Although black is 'cool' it doesn't do much for visibility. For safety sake choose a light color, even better one with some reflective material on it. If you must have a black helmet find some reflective stickers you can put on it to improve visibility.
If at all possible see if you can do a test ride with the full face motorcycle helmet, you may have to leave a deposit or even buy it. You need to do a test ride to see what the wind noise is like. A lesser qualtiy full face motorcycle helmet may have unbearable wind noise problems that won't show up until you exceed 40 mph or so.
And there you have it, you may want to read full face motorcycle helmet reviews as well to help you find the best one. There should also be many helpful links on this page to help you get the best full face motorcycle helmet.
If you're a new motorcycle rider make sure you read my compilation of 101 motorcycle tips for new riders.