Tie back any plants or shrubbery away from the house, so they won't get damaged by the painter's moving back and forth on their ladders. Cover any plants or flowers with a light canvas and make sure the plants have plenty of air circulation. The homeowner will be happy that you didn't get any paint on their plants, or at least that you tried.
Wash dirty surfaces, be it wood, brick, or stucco. You can power wash or hand wash. There are two schools of thought on this subject. One says Power Washing, it's faster and does a better job. The other school of thought is to lightly hand wash the loose dirt. They feel that power washing is too aggressive and that the wood will entrap the water and compromise the integrity of the paint job. I have found both ways to be satisfactory, if you give a couple of days for the exterior to dry from the sun.
Check for any damaged, loose or rotten wood. Now is the time to replace soffit, fascia, and repair rotted wood trim around windows and doors. It's a lot easier to do it now, than after the painting contractor has finished painting.
Scrape and Sand
Scrape all loose surfaces and sand any rough areas. Slight imperfections will remain. Remember, the house is old and so is the wood. Sure, you could have all the wood sanded down to a bare surface, but that would cost a fortune, not counting the health hazards.
A good Painting Contractor should prime all bare surfaces with a good quality exterior primer. To make the paint job easier, your contractor may tint the primer to match the color you have chosen. If you follow these procedures, you'll see that preparation is the keynote to a great paint job.
If you're planning to paint your whole house remember that it'll be a physically strenuous project. It might be wiser to consider hiring a professional painting company to do the work for you instead, as a large-scale project requires a large number of resources, time and energy.