Try to delegate duties and parts of major projects when possible. Not only will this help you manage your workload, but this can be an important aspect of mentoring and preparing other colleagues to step into the chair’s role themselves eventually.
Obviously you’ll want to consider each individual’s strengths and weaknesses when delegating tasks. At the same time, it often happens that a relatively small subset of people are called upon repeatedly. Can you broaden the pool of people who can contribute? There may be individuals who would willingly take on additional responsibilities but don’t think to volunteer, or need a little encouragement. Or perhaps there are individuals who don’t get tapped because of a perceived weakness, but who with a little mentoring could be more effective. Laying out very clear expectations for a given task may be useful as you think about how to delegate responsibilities.