re-filling a form with no model

Probably a basic question here. I have a search form with a bunch of
criteria in checkboxes, selects, text fields, etc. When the form is
submitted I'd like it to be re-displayed with the previous values
(along with search results if any)

I know how to do this with <% form_for @thing %> and assigning @thing to
an instance of something in the controller. But since the search terms
are not tied to a model or any other object, they are just in
params[:terms] for instance - how do you do it easily?