Debugging 'not a valid MySQL result resource' errors?

When you have a 'not valid resource' error, it always means one thing: The variable you are trying to perform an action on, usually db_fetch_array() or db_fetch_object(), does not have a proper result from the mysql query. The reason for this one of the following:
1) You had an error in your query syntax, causing the query to fail
2) You have a query that you didn't actually send to the database

For number 1, this often happens when you are trying to pass a variable to your query, and it's empty. For example:

<?php
$element;
db_query('SELECT * FROM {something} WHERE variable="%s"', $element);
?>

In this case, $element is empty, so the query will fail, even though it looks like it's formed properly.

For example with number 2, you may have done this:

<?php
$query = 'SELECT * FROM {something}';
while($item = db_fetch_array($query))
{
  // do something
}
?>

While it should have been:

<?php
$query = 'SELECT * FROM {something}';
$result = db_query($query);
while($item = db_fetch_array($result))
{
  // do something
}
?>