How best to tell depends on the friend obviously, not two people are the same, and so a canned explaination just wont cut it.
I told my best friend (Whom I have known since age 6) shortly after I found out myself. "Do you know what Aspergers' syndrom is?" I asked him, and it just went from there.
I explained the symptoms. He probably realised where I was going, when I started listing parts of my own personality, because he said "Do you have this?" and I said yeah I did.
It was the happiest moment of my life when I found out that he had researched it throughly after our conversation. Nothing has changed in our relationship though, although I suppose he now has a way of explaining me to his parents
That's a good way to start off. I shall keep that in mind, thank you. I'm glad you were so happy with the result (of telling your friend). I hope you still get on with him well (I'm sure you do because you posted this very recently!