First, how to set semagic up to post to Dreamwidth.
<user name=[lj0] site=livejournal.com>
Embedding videos and suchlike:
Making a block of text smaller
(For those who are curious I have these set up to keyboard shortcuts, so I can type andrewducker, select it, hit Ctrl-Alt-U and get andrewducker automatically. This is basically why I like semagic - ctrl-alt-i for an image, ctrl-b for bold, etc. As a heavy keyboard user it's incredibly useful.)
Original post on Dreamwidth - there are comments there.