When I have to explain when to use "that" and when to use "which" to people who don't grok the rule, I've been using the "Use 'which' when you can take the clause out without changing the meaning" guideline.
I just realize that this could be stated more clearly:
Use "which" when you can take the clause out without changing the meaning or scope of the word that comes before "that" or "which".
Example 1: "Dogs _____ have been spayed or neutered are welcome."
If you take the "have been spayed or neutered" out, the sentence would be "Dogs are welcome". Which isn't entirely true. Not all dogs are welcome, just dogs that have been fixed. Therefore, the sentence requires "that". "Dogs that have been spayed or neutered are welcome."
Example 2: "Dogs _____ are a common pet among Canadian households are welcome".
If you take out "are a common pet among Canadian households", you get "Dogs are welcome". Which is true in this case. All dogs are welcome, and, by the way, they're a common pet. Therefore, the sentence requires "which". "Dogs, which are a common pet among Canadian households, are welcome."
Clauses starting with "which" should be offset by commas. Some sources will argue that this is not necessary, but if you want me to be happy with your work you should use the commas. Your mnemonic for this is if the clause is not strictly necessary to the meaning, you can pick it up by the commas and throw it away.
The commas surrounding the "which" clauses also tend to reflect how we would say the clause out loud. So if you find yourself pausing where the commas would go, consider carefully whether "which" is required.
If you can't remember that "which" goes with disposable clauses, your mnemonic is that "which" has a silent H, which could be removed without affecting the pronunciation (just as the clause could be removed without affecting the meaning.
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