> Another thing he told me was not to be a know it all. One example would be when people said something and I had learned before, instead of diminishing their statement by saying I knew that, I should say something like "wow that's cool" as it makes the person feel better.

Taking this a step further, if you know the topic well you can instead say something like "I know, it's very cool isn't it? Did you also know that <related factoid>".

Delivery is important of course, with the "I know" part needing to be energetic and supportive, but I've found this approach to be an effective way to build rapport and it has lead to some really interesting discussions.