One little apostrophe can be the difference between an error-free card and a grammatical nightmare. In order to make your last name plural, just add an "s." If your last name ends in an "s", then add an "es". Worried you'll mess it up? You can always sign the card from "The (insert your last name here) Family" if you are unsure.
Save the apostrophes for other (grammatically sound) uses. For example, an apostrophe is needed for the phrase "Season's Greetings" in order to make it possessive.