For a Canadian card, put the numbers from your postcode then add zeroes until you get to five digits. I've never tried this myself, but gas stations in Niagara Falls NY will commonly have signs about it.
Can confirm that this works for Canadian credit cards at the gas stations I've used in Washington and Nevada. (In case the parent was unclear: If your postal code is A1B 2C3, your "zip code" is 12300.)
