While I'm generally opposed to creating variables unnecessarily, I tend to be partial to the former of Nico's examples, particularly when an application is in the design stages.
My reasoning is that should a query fail, I like to be able to return the query, as well as any error message returned from MySQL. Ex:
$query = "SELECT foo, bar, baz FROM table WHERE baz='{$_POST['baz']}'";
$result = mysql_query($query, $conn);
if(!$result) {
exit("Query $query failed: " . mysql_error());
}
I find that this greatly helps debugging, especially early in application design, or when initially integrating new queries into an application.
Just my nickel's worth; whatever works for you is cool...