LJ won't let me post the link here (I don't know why, but it doesn't like my href=). You can get the link from this page:
http://www.clanspum.net/~jarrod/NextEntry.html
I also made a "Previous Entry" link. It is also available on the above page.
UPDATE 2006-01-26:
Due to LJ's reorganization of URLs, the scripts needed to be changed. The new versions are at the page linked above. They work for user journals but not communities since community URLs use a different format. If anyone wants/knows how to make a general use one, go ahead and let me know. I'll post yours with attribution.