The only advice I haven't seen yet that you might consider if standing is to wear shoes or some good house slippers. Standing in one place barefoot on a hard surface can really wear out the feet and knees of some people (probably depending on your arches).

A standing mat is good, but gets in the way of the chair if you're trying to sit.

The best solution I have found is simply wear something with padding and support, even if you're a no-shoes-in-the-house kind of person.

Although in my experience, a pair of slippers under the desk will quickly become the favorite sleeping spot for whatever pet you may have.