Editing Complex Pages in Visual Mode

One thing you could do is create page templates for complex pages and simply add the content through the editor, leave as little mark-up as possible in the editor.

Another thing that might help is using the editor style (activated with add_editor_style() in functions.php) and defined in editor-style.css so the client has a better sense of what’s going to be displayed (if you mimic your theme pretty closely) right in the editor.

Add custom fields with descriptive names for information which needs to be displayed in a certain way, in a certain place, rather than lumping it in the main content. The simple fields plugin simplifies this a lot, permits repeatable fields and file uploads. It gives flexibility while avoiding letting your client shoot himself in the foot so easily. Or you could make your own metaboxes,etc.