_form.html.haml

# admin/posts/_form.html.haml

= f.actions do
    = f.action :submit, button_html: { onclick: 'this.form.target = "_self"; return true' }
    = f.action :submit, label: 'Preview', button_html: { onclick: 'this.form.target = "_blank"; return true' }

posts.rb

# admin/posts.rb

# encoding: utf-8

ActiveAdmin.register Post do

  controller do
    def create
      create! do |format|
        format.html { redirect_to preview_or_back }
      end
    end

    def update
      update! do |format|
        format.html { redirect_to preview_or_back }
      end
    end

    private
      def preview_or_back
        if params[:commit] == 'Preview'
          post_path(@post)
        else
          :back
        end
      end
  end
end