1. Adding and manipulating template variables and the Drupal call order:

template_preprocess()
template_preprocess_breadcrumb()
MODULENAME_preprocess() // Defined in module
MODULENAME_preprocess_breadcrumb() // Defined in module
phptemplate_preprocess()
phptemplate_preprocess_breadcrumb()
THEMENAME_preprocess()
THEMENAME_preprocess_breadcrumb()

2. Overriding with Theme Functions and the Drupal call order:

THEMENAME_breadcrumb()
phptemplate_breadcrumb()
sites/all/themes/THEMENAME/breadcrumb.tpl.php
theme_breadcrumb()

3. Devel module and Theme development module is helpful to show you message about theme override.

4. When you theme a CCK filed by adding a file content-filed-field_xxx.tpl.php, do not forget to place an original content-field.tpl.php in the same folder.

5. Do not forget to clear cache if no changes show.