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
Just add it to your Gemfile:
gem 'formtastic-slug', :git => 'git://github.com/johnbintz/formtastic-slug.git'