- Design an application or website inside the browser (no photoshop mockup).

- Find and read some front-end designer blogs.

- Take a good (CSS) framework like HTML5 boilerplate and dissect its code.

- Remake your last favorite designed site without looking at their code. Then compare afterwards.

- Start creating your own framework for rapid prototyping. Add layout-rules to common UI elements: breadcrumbs, pane lists, buttons, forms etc.