Yesterday I ran an impromptu lesson in the Drupal Dojo building on last week's introduction of Druapl 6.0's new theme layer enhancements, namely built-in template files and automatic preprocess_functions(). We covered a topic my colleague Matt blogged about a couple weeks ago: using template files to take control of forms, which is a great way to take your UI to the next level by making it much more designer-friendly.
Check the screencast here: Fine-tuning the UI: Theming Forms With Templates In Drupal 6.0. If you're curious about that technique in version 5, Matt's blog post is a good place to start.
Also, by popular demand I made a short (6 minute) mini-lesson explaining the virtues of devel.module, again in the 6.0 context with the theme_developer tool featured prominently.