Several tools are available to facilitate the edition work of documents on the DocBook format. Due to the complexity of its rules and the large variety of element options and attributes to them, the use of these automatic tools are recommended.
A comprehensive list can be found at the OASIS tools list.
Particularly and without the intention to start any religious "war" among editors, I think the solution emacs + psgml is the best solution at hands, regarding either the open source or the free of charge issue.
The tool used is according to each author's preference, however, it's recomended to have at least the following features:
Analysis of the DTD dynamically
Insertion of elements based on DTD analyzed
Visual highlight of the text either in color or any other possible attributes. This helps to differentiate the text from constant processing elements and instructions on it.