Diversify the drive-in experience. Make it a multi-purpose venue; movies, concerts, a mini-putt, an arcade, so on and so forth. Cross the drive-in theater with the drive-in eatery; have food (actual food as well as snack bar fare) delivered on skates. Sell it as an upgrade from the indoor theater: No sitting next to smokers and loudmouths and crying babies, no getting stuck to the floor because Billy spilled his root beer and Milk Duds, you can decide how loud you want the sound, and if your kids get fidgety and restless, you don't have to take them out into the lobby.
Eventually, the indoor theater is going to win because of the better visual and sound technology that won't be as easily enjoyed in a car hundreds of feet from the screen, and the private experience selling point will go away too with the advent of cable, VCRs and eventually streaming media on increasingly bigger and better televisions. But it can still maintain a niche, particularly in resort areas where coin-op video games and fudge sellers are still bountiful.