Early blight is pretty much impossible to completely stave off, but you can certainly delay the onset and slow spreading. The most important preventative measure you can take is to prune all leaves, branches, and suckers within about a foot of the ground. The disease starts when dirt containing the fungus splashes up on foliage, so if you can avoid that, it'll go a long way toward keeping the plant healthy longer. Combine that with a normal Daconil type fungicide, and fertilize with Neptune's Harvest or some other slow-release form of nitrogen to ensure that new growth keeps pace with the dying lower vegetation.