On a server with a lot of users, the log file can get very big. It's possible to archive the log.nsf file, but it doesn't reduce the file-size, and it's most of the time impossible to compact the log.nsf while the server is running.
The solution as proposed by IBM: shutdown the server and compact the log file manually via ncompact.exe
I'm proposing a new system for log file archiving, similar to how mail journaling is handled in Domino:
After a certain period, or when the log reaches a certain size, move the log to a another location and create a new empty log.nsf file.