As a programmer you may need to build a website that can be easily edited by your client. It is important to understand from the outset what they want to be able to edit and add / remove. Some clients will just want to be able to make text changes and others may need to be able to upload new banners, new products, and new categories etc.
Whether you are using an off the shelf content management system or a bespoke one, you will need to ensure that you build it in such a way that they can add their own elements to the site where needed and that it is easy to use. Not everyone has any knowledge of code and how it works so they will need to be given text boxes, drop downs and image upload sections to help them complete it.
Always give your client full training before the site goes live and make sure that regular back ups are taken. If possibly create a simple screen by screen user guide for them to allow them to look back if they forget or get stuck whilst trying to make a change to the website.