How do you deal with rude people while driving?
Use it as a learning experience. You don't say why this person flipped you off, but if it was because of something you did while driving, use it as a reminder to drive more carefully.
If you did nothing wrong and he was just being unreasonable, take comfort in the fact that you are unlikely to see him again and he’ll probably forget all about you before long.
Either way, try not to dwell on it. The unfortunate reality is that we are all going to meet rude people in life. The blessing is that there are a lot of nice and courteous drivers out there as well!
Rather than dwell on the negative, try to remember some positive interactions you’ve had with other drivers. Then, keep on being one of the nice ones!