A funny RSVP response card has a better chance of being returned. At least, that's why most people send them. After all, which would you most likely send back -- a formal, conservative card, or one printed in big red letters with choices like: "The horn sounds! The horde will ride"? A funny response card also signals to your guests that your celebration, be it a simple get-together or a wedding, is definitely not going to stand on ceremony -- and will probably be a lot of fun.
Weigh your words before you compose your response cards. Will your guests understand and appreciate the joke? You want to amuse your guests, not offend them. Be sure to avoid response choices that imply guests might be showing up just for the food, or don't really have a valid reason not to attend. You know you're joking -- but some of them might not.
Tailor the style of your responses to your party theme. If you're inviting guests to a Hawaiian luau, compose responses like: "The big ship arrives; bring flowers -- man the canoes!" Or: "The volcano trembles. You must go on alone!" If it's a Western-themed wedding, choose responses like: "We'll be there with bells on! Git ready for the charivari!" Or: "Sure am sorry. The Clanton gang's in town and ain't nobody comin' out 'til Marshall Dillon gets back."
Consider the design of the response cards. Will they be funnier with bright colours and big, funky fonts, or will they be stealth jokes? Sometimes a proper-looking card can be very funny if its wording is wildly at odds with its strict lettering and prim design. Picture the impact of a wedding-grade response card that on closer inspection reads: "The Mother Ship at last! Prepare to beam up!" Or: "It's a trap! To the escape pods!"
Relax. If you've judged your guests' sense of humour even reasonably well, they'll be delighted with your cards -- and hopefully, will respond something like: "The visitor is on his way. Repeat, the visitor is on his way."