The phpBB Template Design Guide was updated today and the last two parts were added. Parts four and five cover some common questions, problems, general advice as well instructions on how to export, distribute and support your template. I've tried to share the things I've learnt over the years I've been working with phpBB and designing as well as releasing templates.
The guide is work in progress so if there's something you miss, let me know and I'll look into including it.
http://www.jakob-persson.com/articles/tutorials/phpbb/templatedesignguide