I've also been playing with Page2RSS which works really well: copy the LJ post URL, go to http://page2rss.com/ and paste it in. There's also a bookmarklet there. Then subscribe to the feed in your RSS feed reader and you'll get a really nicely formatted update every time someone adds a comment. At least for small threads; I'll have to play more to find out what happens once the threads get complicated and/or go past the first page.
Any other ways?