Every painting job develops a unique choreography as ladders go up and come down and tarps are unrolled and folded up. But two basic principles remain: 1) Start at the top and work down. 2) Work in the shade, out of the sun's glare. As the dance proceeds, keep an eye on the weather. Rain can wash freshly applied latex right off the wall, and a temperature dip below 50 degrees F two days after application can interfere with adhesion and curing and dull the sheen of glossy paints. (Latexes like Sherwin-Williams's Duration and Benjamin Moore's MoorGard Low Lustre are formulated to tolerate temps as low as 35 and 40 degrees, respectively.)
Don't hesitate to ask all the questions you need to feel comfortable hiring a professional. You can also ask 3-4 professionals to make sure you have the right quote in mind. You must make sure they have a warranty or some kind of paperwork stipulating the quality of their jobs. Insurance is also a #1 priority to consider, as damage can happen in the process of painting.
Before the scrubdown, protect nearby plants by misting their leaves and saturating the surrounding soil with water, pulling them away from the house, and shrouding them in fabric drop cloths. (Plants will cook under plastic.) Lay more drop cloths along the base of the walls to collect any falling paint debris. Walls should be wet down before getting scrubbed, then washed with a gallon of water mixed with 1 cup chlorine bleach and 1 cup of either a concentrated, phosphate-free cleaner, such as a trisodium phosphate (TSP) substitute, or Jomax House Cleaner. Working in sections, from the bottom to the top, will avoid streaks. Be sure to rinse walls well before the solution dries. Wood siding and trim should be ready to paint after a day or two of dry weather. Lafayette House Painting