to the right of the YouTube screen there is a box, that contains a line of text entitled "embed". It is simply a matter of copying and pasting it into your post. The last Alan Partridge embed text looks like this:
:
<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/n5jpVbEL0jc"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/n5jpVbEL0jc" type="application/x-shockwave-flash" wmode="transparent" width="425" height="350"></embed></object>