I was going through one of my projects and I came to know that how simple it is to insert additional information into your site's search indexes using hook_nodeapi. A comprehensive details about hook_nodeapi can be found here.

Following are the steps involved to make this happen:

• Create a module that implements hook_nodeapi, lets say example_nodeapi()

function example_nodeapi($node, $op) {
  // Function definition here
}

• Add logic for node operation "update index"  in hook_nodeapi function

if ($op == 'update index') {   
    $keywords = array();
    // Implement logic to parse $node->body to get desired keywords and
    // add one by one in $keywords array.
    if (isset($keywords)) {
      return implode(' ', $keywords);      
    }
  }

• Full hook_nodeapi implementation for "update index" operation on $node will look like 

  function example_nodeapi($node, $op) {
    // Function definition here

    if ($op == 'update index') {   
      $keywords = array();
      // Implement logic to parse $node->body to get desired keywords and
      // add one by one in $keywords array.
      if (isset($keywords)) {
        return implode(' ', $keywords);      
      }
    }
  }