Validation is the difference between one-upping and contributing to the conversation. How do you validate someone else's story with your own? By finding a common thread. For example:
Him: My plane was stuck on the runway for 6 hours!
You: I once sat on a runway for 7 hours!!
You've added nothing to the conversation, and you've belittled his experience.
Him: My plane was stuck on the runway for 6 hours!
You: I hate that. Especially how they won't let you get up to use the bathroom.
Him: No kidding! I was holding in a wicked deuce the whole time.
You: That's terrible. One time, I had to poop in an airsick bag.
It's a subtle difference--you're still "one-upping" the other guy's story. But by first engaging a shared detail, you've validated his experience. Of course, the more details you can engage, the more validation you'll provide.
Him: My plane was stuck on the runway for 6 hours!
You: I hate that. Especially how they won't let you get up to use the bathroom.
Him: No kidding! I was holding in a wicked deuce the whole time.
You: That's terrible. And did the pilot keep making bullshit announcements?
Him: Yeah! Every five minutes--"we'll be taking off shortly".
You: That pisses me off so much. Just be honest with me, you know?
Him: Passengers were getting angry, dude. It was tense.
You: One time, I was on a flight that was delayed for 7 hours, and this guy punched the flight attendant, and an Air Marshall shot him four times, and while everybody was distracted, I pooped in an airsick bag. I'm on the no-fly list until 2040.
You've one-upped the shit out of his story, but it's in the context of a conversation about the shared details of the experience, so you don't look like such a dick.