Unable to clear text field with overridden submit method used for ajax implementation via Jquery 1.4

问题

I have a functionality of posting messages to a group which works using AJAX via Jquery. When I click on the "Post" button which is the value of the submit_tag, javascript is rendered and the page without reloading displays the latest message posted. Basically the post_message.js.erb file is called. I am making use of the following tutorial to implement the same:- http://railscasts.com/episodes/136-jquery

The code for post_message.js.erb looks like this:-

$("#new_post").before('<div id ="new_post"><%= escape_javascript(flash.delete(:notice)) %></div>');<!--TO-DO the flash messages aren't yet enabled/working-->
$("#latest_post").prepend("<%= @group_post.message %> by <%=  Investor.find(@group_post.post_by).first_name %> <div class=contentdispgrp> <%=  distance_of_time_in_words(@group_post.created_at,Time.now) %> ago</div> <br/><br/><hr/>");
$("#post_msg")[0].reset();

The part of the form which would otherwise render HTML and display the posted message with reloading the page looks like this:-

<%form_for  :group_post, @group_post, :url => {:action => :post_message, :id => params[:id]},:html => {:multipart => 'true', :id => 'new_post'},:id => 'post_form' do |f| -%>
    Start Discussion:<label id="post_msg"><%=f.text_field :message%></label>
   <!--<%=f.file_field :photo%> -->
   <%=submit_tag "Post"  %></p>
<!--<input type="button" value="Reset Form" onClick="this.form.reset()" />-->
  <%end%>

<div id = "latest_post"> </div>
<%for a in @group_all_posts %>
<%= a.message %> by <%=  Investor.find(a.post_by).first_name %> <div class ="contentdispgrp" id="style_chck"> <%=  distance_of_time_in_words(a.created_at,Time.now) %> ago </div><br/><br/> <hr/>
<%end%>


</tr>

I want to empty the text field also along with the above already implemented functionality. I guess , I need to some how make use of the Javascript reset method , but somewhere I am not using it correctly.

Kindly help me with the same.

Thanks for your help..

[EDIT]

Code for post_message in my groups controller.

def post_message 
      @investor_group = InvestorGroup.find(params[:id])


      unless @current_user.is_an_existing_member_of_group(@investor_group)
        flash[:notice] = "Please join this group to participate in discussions"
        redirect_to :action => :show, :id => @investor_group and return # try to find out what exactly does this command do with return stmnt
      else
        @group_post = GroupPost.new(params[:group_post])
      end
      #@group_post = GroupPost.new(params[:group_post])

      investor_id = session['investor_id']
      @group_post.investor_group_id = @investor_group.id
      @group_post.post_by = investor_id
      if @group_post.message.blank?
         flash[:notice] = 'Post can\'t be blank.'
      end
      if @group_post.save
        flash[:notice] = 'Post was successfully created.'
       # redirect_to :action => :show, :id => params[:id] - removed after trying to implement ajax via jquery
      else
        flash[:notice] = 'Post was not successfully created.'
      end


      respond_to do |format|
        format.html {redirect_to :action => "show", :id => params[:id]}
        format.js
      end

  end