Views 3 for Drupal 7 kindly gives each exposed widget DIV a class with a unique name. Then it's a matter of adding clear and float attributes.
If you're using Views 2 you will have to override the views-exposed-form.tpl.php file in the views/theme folder. You can just copy and save this code in a file called views-exposed-form.tpl.php in your theme file path. It should override the original template file.
<?php
// $Id: views-exposed-form.tpl.php,v 1.4.4.1 2009/11/18 20:37:58 merlinofchaos Exp $
/**
* @file views-exposed-form.tpl.php
*
* This template handles the layout of the views exposed filter form.
*
* Variables available:
* - $widgets: An array of exposed form widgets. Each widget contains:
* - $widget->label: The visible label to print. May be optional.
* - $widget->operator: The operator for the widget. May be optional.
* - $widget->widget: The widget itself.
* - $button: The submit button for the form.
*
* @ingroup views_templates
*/
?>
<?php if (!empty($q)): ?>
<?php
// This ensures that, if clean URLs are off, the 'q' is added first so that
// it shows up first in the URL.
print $q;
?>
<?php endif; ?>
<div class="views-exposed-form">
<div class="views-exposed-widgets clear-block">
<?php foreach($widgets as $id => $widget): ?>
<div class="views-exposed-widget views-widget-<?php print $id ?>">
<?php if (!empty($widget->label)): ?>
<label for="<?php print $widget->id; ?>">
<?php print $widget->label; ?>
</label>
<?php endif; ?>
<?php if (!empty($widget->operator)): ?>
<div class="views-operator">
<?php print $widget->operator; ?>
</div>
<?php endif; ?>
<div class="views-widget">
<?php print $widget->widget; ?>
</div>
</div>
<?php endforeach; ?>
<div class="views-exposed-widget">
<?php print $button ?>
</div>
</div>
</div>