"Define before you design." This statement holds true for many web applications. For example, when using Adobe Dreamweaver, you will want to define a site before you design the site to make it easier for Dreamweaver to manage the files it created for you.
"Create it, relate it and then populate it." While teaching on how to create a database, I often simplify the process by telling my students, you create the database tables, relate the database tables and then populate the database tables.
"Every tag is a container." It is helpful to think of every tag as a container when working with markup languages like HTML and XML.
"Object, object, object everywhere." It is also helpful to think of most things you work with as objects and that objects have properties. For example, if you think of the <h1> tag as an object, you can intuitively think of properties of the heading object (e.g., font size, color, font type, etc.).