It's pretty simple once you get the hang of it!
The code below stays the same EXCEPT for the portion in bold.
So let's say you wanted to post this: http://youtube.com/watch?v=cVtAZi4VVY4
You'd use this code:
<p>
<object data="http://www.youtube.com/v/cVtAZi4VVY4" height="300" type="application/x-shockwave-flash" width="380"><param name="movie" value="http://www.youtube.com/v/cVtAZi4VVY4"></param></object>
</p>
In the post editor you need to stick this code in the HTML pop-up, above the smiley option.
...You my friend, I will defend.
And if we change, well I love you anyway.