does anyone know how to put short video clips in an entry for livejournal

What type of video? AVI? As far as I know, LJ doesn't support video, at least not for free accounts. I could be wrong. You could convert it to a GIF though and put it in just like an image..

However, keep in mind that this can slow loading times really badly for the poor people viewing their friends pages. Why not just link directly to the video?

I don't believe LJ supports embedding of video files.