Like many things in mobile and digital marketing, the mobile app market moves fast. While the very top of the app stores are consistently dominated by mobile-first games and apps - and those with large web user bases migrating to mobile - the top 100 overall and top charts in each category are dynamic. Regularly reviewing the mobile app stores - specifically Apple’s App Store and Google Play - can provide insights and ideas for improved user interfaces (UI), user experience (UX), monetization and more. Below is a starting point. You may develop your own process for reviewing the app market as makes sense for your app or portfolio. To help illustrate a topic we’ll take the perspective of a publisher of a photo-sharing app.
Even if the top 10-20 spots on the free app charts stay remarkably consistent, there is always that 1-2 apps that either shot up out of nowhere, are new or have found their audience and jump into the top 10 or 20. For example, the makers of Candy Crush have their newest app in the top 10 on google play - Blossom Blast. 
Knowing the latest Candy Crush title is in the top of the free charts in unlikely to lead to some obvious action for your app. What is actionable is watching which social networks are rising, or looking for new tools from existing networks (like Facebook Messenger). A new social network may affect how users can share their photos in your (hypothetical) app. The top free app charts is a quick and easy way to get a snapshot of the mobile app market.
This is where it gets fun. The "Top Free" charts are interesting, but which apps are making money? It is important to remember here, that top grossing refers to in-app purchases (or paid apps), and does not take into account advertising revenues, web-based subscriptions, or purchases made outside of the app store (the Amazon app for example). Review the top grossing charts for overall and by categories related to your app as a monetization method in an unrelated app may spark ideas for your app. With a photo app, understanding the basics of game monetization/gamification is probably all that is needed, and our research of the top grossing can skip most games. The top 100 grossing is mostly games, so if nothing relevant jumps out, move right into the top grossing for your app’s category. Note that top grossing by category is only available in the Apple App Store. 
No matter how your app monetizes, it is important to understand how other apps that share a category (and likely a user base) monetize their apps.
You probably have a specific awareness of your top competition in the app stores, and a general knowledge of those apps that compete with your app for specific search terms. In fact, a regular review of competitors and related apps is likely (or should be!) part
of your app store optimization process. A competitor’s move up the app store may be related to specific features released in its latest version.
That is to say - category rankings and keyword rankings are trailing indicators of an app’s performance. Create a routine for reviewing your top competitors’ mobile app offerings, new features and recent reviews. 
Gummicube clients can access category and keyword competitors, their trends up or down the charts and ratings and reviews from their app’s dashboard.
Apple started displaying “trending searches” in the search tab of the mobile app store - which is interesting by often not very relevant. 
Gummicube has our own index of app store data, across Google Play and Apple’s App Store. This data provides mobile app publishers and marketers access to trending searches overall, trending searches by category and trending searches related to a specific search term. Trending keywords analysis should be part of every app store optimization process. Monitoring keywords and trending keywords provides early visibility to new competitors.
Once you have a routine for these basics, adding review mining, competitor ad campaigns, competitor SDKs used in their app etc.. can provide further insights. Monitoring the app stores, and zeroing in the changes that are important to you and your app is made much easier with software. Take a look at how Gummicube uses app store data for research, app store optimization and user acquisition by requesting a demo here.
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Developers on the iOS App Store should plan in advance of the upcoming Holiday Schedule to allow enough time for apps to get approved during the busy holidays.
App Store Optimization is an involved process that should be regularly revisited based on recent changes in trends. Iteration is one of the key drivers for success in ASO.