Another strategy you could try is to figure out why people like your App 1 (game?). You'd have to build in some sort of stat collection. Then you can try to replicate that in your later apps.

For example, if you look at games published by Donut games they all have the same basic structure. A bunch of puzzle levels on a 3 star rating system, and you have points assigned to you as you finish the level in the optimal fashion. The game itself is different every time but most of the time you're picking up a point object as you traverse the level, or you're traversing something in the shortest path. Whatever it is about these games, my girlfriend has downloaded almost all of them. You could try to go for the same effect.