I use the dust brush attachment on my vacuum...

Vacuum brush works well. Or you could hire a professional to do the first big cleaning push.

We bought an old, small log home which needed significant cleaning due to years of non-use, smells, and potential hantavirus (from mice). Before we moved in I cleaned every darn log in that place with a scrub brush, some multi-purpose cleaner (like Mr. Clean), and hot water. This may not have been the correct way to clean the logs, but it worked, and did not do any damage to the logs at all.