There are only a few rules for writing DocBook code that you plan to process with dbslide. Some of them are more or less trivial, but I will list them here, just in case:
The whole presentation is a book, and every single slide is a chapter.
Keep your words to a minimum. Otherwise your message does not fit on a slide, and people can't read more than 10 words per slide anyway.
Don't try nesting headings. You don't have that much space anyway. Stick to the simplesect element for all your heading needs.
Avoid elements that add verbose captions. E.g. informaltable and informalequation are more suitable than table and equation, respectively.
There is no exact way to tell when a slide is "full" besides transforming the document to the desired output format.