Keep moving. Black flies may follow a moving target, but they rarely land and bite. This is a good excuse to avoid gardening and yard work and do something more active.
Wear light-colored, full-coverage clothing. Black flies are attracted to dark colors. Unlike mosquitoes, they can;t bite through clothing.
Whenever possible, keep to open areas and higher ground; black flies disperse in a breeze. This is a good excuse to climb a mountain.
Get out early in the day: black flies are generally inactive until the air temperature has risen to at least 50 degrees.