to augment bravehearts excellent answer, check out 2-way humidity packs, too. 55-58-59-62% options available. integra boost or boveda. can only hold so much and can only lose so much h2o, so there are limits to them. As long as properly dried before using, they are awesome at maintaining RH.
don't pick this answer, please. it is only a useful tangent to the original question.