How do you prefer to organize the code of a react+Redux App?

Here's how I currently tackle it, based on the Ducks pattern :

/components
  /shared
  /area_of_concern
/ducks
  /area_of_concern
    /actions.js
   /index.js
   /reducers.js
   /operations.js
  /utils.js
/shared
  /various_utility_files.js

A few other notes:

• The areas for concerns between redux and components don't have to map 1-to-1. Your data structure does not always map to your UI in that manner.
• operations are functions that either have side effects or dispatch multiple actions. Thunks live here if you are using those.
• Utils are complex functions and hooks broken out of components. This makes them more composable and easier to test.
• Keep test files in the same folder as the code they are testing. Kent C Dodds has a great article on colocation as a guiding principle for organizing files.
• Favor components with longer names over a complex folder structure. I would not go deeper than the above example without a very good reason.

@markerikson (Redux maintainer) recently posted a Redux Style guide.

It's contains best practices, patterns, which can guide you to organizing your Redux site.

FYI - Structure-wise, the guide recommends, Feature Folders or Ducks pattern.

public
-- index.html
...

index.js

src
-- assets
--- css
---- common.css
...

--- images
---- image1.jpg
...

-- layouts
specifies common layouts, renders routes and matched views
--- layout1
---- layout1.js
---- layout1.routes.js

-- views
(usually rendered by a route and responsible for component layout)
--- view1
---- view1.js
---- view1.reducer.js
---- view1.action.js
...

-- components
(included in views)
--- component1.js
--- component2.js
...

-- services
(common code, authenticator, rbac component provider)
--- service1.js
--- service2.js
...

-- lookups
(application constants/ config logically grouped together)
--- lookup1.json
--- lookup2.json
...

This is a general structure I follow. It aggregates most of my code in a logical and easy-to-find way. Mostly came across this on need basis.

Also, I am using a redux middleware only for api calls which manipulate the application state (shared by more than one component). Otherwise, I use my custom made AsyncRequest and AuthenticatedAsyncRequest to reduce redux's boilerplate code.

I would suggest you to look into React Suspense (just came across it, pretty cool but i think it's experimental).