<!-- BEGIN COPY / PASTE -->
    // Add custom fields to user profile
    function my_custom_user_profile_fields($user) {
      ?>
      <h3>Extra Profile Information</h3>
      <table class="form-table">
        <tr>
          <th><label for="twitter">Twitter</label></th>
          <td>
            <input type="text" name="twitter" id="twitter" value="<?php echo esc_attr(get_the_author_meta('twitter', $user->ID)); ?>" class="regular-text" /><br />
            <span class="description">Please enter your Twitter username.</span>
          </td>
        </tr>
      </table>
      <?php
    }
    add_action('show_user_profile', 'my_custom_user_profile_fields');
    add_action('edit_user_profile', 'my_custom_user_profile_fields');

    // Save custom fields
    function my_save_custom_user_profile_fields($user_id) {
      if (!current_user_can('edit_user', $user_id)) {
        return false;
      }
      update_user_meta($user_id, 'twitter', $_POST['twitter']);
    }
    add_action('personal_options_update', 'my_save_custom_user_profile_fields');
    add_action('edit_user_profile_update', 'my_save_custom_user_profile_fields');
    <!-- END COPY / PASTE -->