Money is the key, both at the top of the purse structure but also through the field. Traditional sprint cars thrive on numbers, just look at the crowds that fill tracks for IWS. Fans like heats that have compelling and competitive races to advance. They like and C and B mains with good drivers and cars fighting to make the A main. Pay through the field in the A and better funded teams and drivers will stay with the series making the entire night more exciting. Pay through the field and make the payout reflect a profit on the night for anyone who makes the A (notwithstanding damaged cars) and the entire program becomes more exciting. It keeps the crowd entertained and it draws cars.
And 1 more thing, keep the program moving. The sprint main should flag green no longer than 2.5 hours from the beginning of the program. Any track contracted will only be allowed 1 support series. The support series runs a heat or B after sprint qualifying, sprint heats, the sprint C main, the sprint B main, and the sprint feature. The support group fills down time only and has a time limit each race.