Frontier is currently running a promotion offering 12% off on domestic fares booked from now until 11:59 PM EDT on Thursday, July 17th, 2014. The discount is valid on fares booked with travel dates from August 20th through December 12th, 2014.
In order to take advantage of the promotion you need to enter the promo code HITTHEBOOKS when you make your reservation online at FlyFrontier.com. There are a couple blackout dates that you need to be aware of. The offer is not valid for travel on November 21st-26th nor on November 29th-December 1st, 2014. Be sure to read the full terms and conditions on the promotion page before you book.
Keep in mind that Frontier is an ultra-low cost carrier comparable to Spirit and Allegiant meaning you will be charged for any little extra item including a pop or snack. However, unlike Allegiant, you will not have to pay for carry on bags as long as you book via Frontiers own website (which you will have to for this offer anyway).