The simplest way (but also the least customizable) is to add <%= error_messages_for 'objectname' %>
within your view. In your action you need to stick something like render :action => 'pagename' in your controller, where 'pagename' corresponds to the view containing the error message helper.
If/when you need to tweak things slightly, try something like
<% unless @post.errors.empty? %> The post couldn't be saved due to these errors: <ul> <% @post.errors.each_full do |message| %> <li><%= message %></li> <% end %> </ul> <% end %>
in your view- more on this <a href="http://wiki.rubyonrails.org/rails/ pages/HowtoValidate" title="here">here</a>.