Communicate with the line of business to identify their most critical roles. What will they be looking for as business strategy changes?
Build relationships before a need arises. Seek out your 'dream list' of top talent and start building relationships with them so that when positions open, it's a warm call to candidates.
Know who is being developed internally for critical roles and how soon they will be ready to be promoted. Keep abreast of any changes (such as if that person leaves the company) so that you aren't caught off-guard if the role opens up and an external hire is needed.