My only opinion is that stressing about it is sort of useless - it'll go well or it won't or be inconclusive - and stress will make you, if anything, act in a non relaxed way.
Do you have an idea of what sort of person you want to him, or anyone, to see you as?
I have my own memories of seeing a guy I had a crush on, that I had met only slightly, as he worked in a different part of our building. As we approached each other in the major first floor hallway, I started to turn bright red, all my interest showing in my face. I can't remember what I said, probably just hi, and kept walking. I was an adult by then, and this was like being thirteen again. I was a mess of feelings, being horrified at myself for blushing and being glad to see him. Not long later, we started dating. So even what I thought was the worst possible thing happening, my turning red, turned out ok.
Maybe breathe deeply to calm down when you do meet him - but I'm no expert on the wisdom of that. I do it when I am all freaked about something, and it helps me.