Preparation is key!
As with any decorating project, preparation is the single most important aspect of exterior decorating. While it can be tempting to rush this step and get on with the painting, putting time into your preparation is the best way to ensure a perfect finish that lasts.
Make sure you thoroughly clean down the surface before painting, as they can easily accumulate dirt over time. If left on the surface, this dirt can impact the ability for the paint to stick to the surface. Wash off any dirt with hot soapy water or even better a pressure-washer.
If there is any organic growth such as lichens, apply a fungicidal wash which will kill the growth. Any loose paint from previous decorating must be removed before you can paint the surface. Use a filling knife to remove the paint back to a firm edge, where no more paint can be removed.
It is also important to fill any surfaces that have been damaged before you start painting. Any cracks or holes can allow moisture to get behind the paint film, so filling is a sure-fire way to watertight the surface.