• Add jQuery to the page using the method described in this post
• Download the jQuery Hint Box plugin and link it to the page. 
• Call the script and set the options for the position and html content of the hint. Here is an example:

<script type="text/javascript">
$("input[title='Author']").inputHintBox({
	className:'simple_box',
	html:'Who is the author of this quote?',
	incrementLeft:-25,
	incrementTop:-15
	});
</script>

The above script simply selects  the field using its  title attribute then sets the css class for the element, its contents and position.

Here is the CSS I used for my example.

.simple_box{
	border:1px solid gray; background:url('/_layouts/images/toolgrad.gif');
	font-size:xx-small; color:#444; padding:7px;
}

Here it is in action.

hints

Related posts:

1. Improving #SharePoint Forms – Character Count
2. Hiding the user presence icon
3. Improving #SharePoint Forms – Watermarks