Special considerations for Drupal 6?

Hi guys,

I've been swotting up on the theming introduction and currently plan to base my new PHPTemplate theme on Zen.

I will be theming a test Drupal 5.1 installation I have, but the site won't really go live for at least a year. With this in mind, can anybody give me any advice or tips to make my move to Drupal 6 as painless as possible?

Thanks a lot!

Lewis.