i've flown with spirit for the last 4 years and i have never seen a promo code or coupon. they always have super lows fares though and i recommend signing up for their "syber specials" email
here. also right now they have some groundhog day specials.
"Forget Groundhog Day, let’s celebrate Airhog Day with 6 more weeks of low fares," said Tom Anderson, senior vice president and chief marketing officer. "We’re having some fun today celebrating Groundhog Day in our own way. We hope that customers will visit Spiritair.com and make plans to spend part of their winter traveling with us."
Spirit Airlines, the low-fare leader, is celebrating Groundhog Day with sale fares starting at $44*. Fares need to be booked by February 10, 2005 and are available for travel February 22, 2005 through April 30, 2005, and are valid Monday/Tuesday/Wednesday southbound, and Wednesday/Thursday northbound. Blackout dates: 3/23/05 – 4/3/05.
I see you are from michigan so here are the Detroit deals...
Detroit
Detroit to/from Atlantic City from $64* each way
Detroit to/from Cancun, Mexico from $144* each way
Detroit to/from Los Angeles from $108* each way
Detroit to/from Nassau, Bahamas from $124* each way
Detroit to/from New York LaGuardia from $62* each way
Detroit to/from Providence (Boston Area) from $49* each way
Detroit to/from San Juan, Puerto Rico from $124* each way
Detroit to/from Santo Domingo, Dominican Republic from $149* each way
Detroit to/from Washington, DC Reagan National from $47* each way
Good luck!!