Rather than distribute hardcopy updates to outside counsel guidelines (environment forbid!) or even tediously email them items that have to be updated, move into the 21st century. Insert where appropriate in your guidelines language to the effect of “We expect your firm to check our online update to these guidelines” and include a URL to a site where you publish changes (See my post of July 11, 2008: guidelines for outside counsel with 16 references.).
One situation for this technique is updates on policies regarding disbursements (See my post of Dec. 1, 2006: disbursements of law firms with 7 references.). Another is a list of corporate entities for firms to use to check conflicts. A third could be revisions to budget forms or task codes.