Saving data to the database after form validation - best practices?

[eluser]mddd[/eluser]
Thanks bretticus and WanWizard.

@bretticus: Good point, I have to make sure the data passes through the xss filter somewhere along the process.

@WanWizard: Nice overview, thanks. The main idea looks a lot like what I usually do too. As I am starting on a new project now, which has a few forms (like most..) I thought I'd make it more structured and more reusable.
Question about the update_from_post part: do you use the form_validation library as the basis for this (e.g. walking along the fields set in form_validation) or do you provide the field list some other way? And do you use set_value etc. or take the values form $this->input->post?