Try to load the script in the footer section.
<script type="text/javascript"
src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script type="text/javascript"
src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.18/jquery-ui.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("#clientAutocomplete").autocomplete({
source:'client_autocomplete.php',
minLength:1
});
});
</script>
This worked for me