The embarrassing buildup that some people refer to as belly button wax is a combination of lint fibers and dirt that accumulates over time. Some studies indicate that as many as 67 different bacteria types nestle in the navel as well.
Regular cleaning of the belly button is the best way to keep this greasy buildup from developing. A quick scrub of the navel while in the shower should work for most cases. If bacterial development is a concern, dabbing a ball of cotton in some rubbing alcohol and rubbing the cotton around the navel once a day for up to a week eliminates most problems.