Now that prototype support the css selector (version 1.5.0pr0), we can use it to defines rules to apply to certain elements as with behaviour.
more than long words here is a simple exemple.

<script>
/** 
 declare a behaviour object to help you manage behaviour stylish rules
 use register to register rules at page loading 
 use reload to re-apply rules on element updated after an ajax.update
*/
 var Behaviour = {
    register: function(rules){
      this.rules = rules;
      for(rule in this.rules){
        this.reload(rule);
      }
    },

    reload: function(rule){
      elements = $$(rule);
      for(y=0;y<elements.length;y++){
        this.rules[rule](elements[y]);
      }
    }

  }

/**
 set your behaviour rules here
*/
myrules = { '#hiden': function(elmt){ Event.observe(elmt,'click',function(e){ Element.hide(this);})
                                                                                },
            '.showhiden':function(elmt){     
                                                        elmt.style.cursor='pointer';
                                                        Event.observe(elmt,'click',function(e){
                                                                                                                Element.show('hiden');
                                                                                                             });
                                                    }
             };
window.onload = function(){ Behaviour.register(myrules);};
</script>
<div id='hiden' style='border:dashed red 1px;display:none;'>this is some hidden text</div>
<br>
<span class='showhiden'>show hidden text</span>

This is not an official way for doing this, but can’t find any ‘official’ exemple of using behaviour with the last version of prototype. This method works fine so why wait to use it :p