Here in the Quad Cities paint will not adhere to dirty surfaces, so make sure you wash your house siding and trim thoroughly and allow to completely dry.
Make sure to wear a dust mask before scraping and sanding your homes exterior to avoid lead based paint once all peeling paint is removed you need to use an oiled based primer to create a sound working surface for the exterior paint to adhere to.
When Painting the exterior of your home always start painting at the top of the house and work your way down, this will cause you to work with gravity than against it and help when you inevitably get drips to feather them out and keep a professional looking exterior paint job.