The last big decision is how to apply the paint. Most pros use paint sprayers because they're fast, but in inexperienced hands a high-powered sprayer can leave drips, thin coats, and a mist that may land on many things other than your siding. If you do hire a painter who uses a sprayer, make sure he is meticulous about removing, covering, or masking off everything in the area that might get hit with overspray: gutters, roofs, windows, shrubbery, walkways, cars—you name it.
Once you choose a paint brand, the pros advise against additives, such as mildewcides. But they will add conditioners such as Floetrol (for latex) or Penetrol (for oils) to slow drying times in hot weather so brush and lap marks don’t show, and to make paint more sprayable.