Yoni Heisler, BGR:

One of the great things about Face ID is that the data associated with your initial Face ID photo is always being updated to account for even subtle changes in your appearance. That being the case, there’s an incredibly simple way for iPhone X users to train Face ID to work flawlessly across all situations. So if you’ve been noticing that Face ID works great 99% of the time but seems to slip up if you hold up your phone at an angle or from a new distance, we’ve got a solution for you.

And:

The next time you try to unlock your iPhone X with Face ID and it doesn’t take, don’t try to unlock it with Face ID a second time. Instead, enter in your passcode. Doing so effectively tells Face ID to incorporate facial data from whatever new angle or position you happen to be holding your phone in. That being the case, the next time you attempt to unlock your phone from the same position, your phone will unlock immediately.

This is a terrific tip.