Add a slug field, that generates a slug based on another field.

= semantic_form_for @blog_entry do |f|
  = f.input :title
  = f.input :slug, :as => :slug, :based_on => :title

You can put in the list of existing slugs and the editor will avoid using those:

= semantic_form_for @blog_entry do |f|
  = f.input :title
  = f.input :slug, :as => :slug, :based_on => :title, :existing_slugs => @blog_entry.class.slugs

Just add it to your Gemfile:

gem 'formtastic-slug', :git => 'git://github.com/johnbintz/formtastic-slug.git'