Explicitly assign the responsibilities of the system to appropriate modules.
In a web context, an application is made up of layers, each being closer to the end-user:
Domain (farthest from the user)
UI (closest to the user)
The crucial rule in this context is the arrow of dependency. It states that a layer can never import or mention anything from another closer to the user.
This approach is a powerful concept and the key to a maintainable and scalable codebase.
My advice: never compromise on architecture. It's fine to be pragmatic and take shortcuts in any other areas. But you need a solid foundation for your system to grow and scale. It's virtually impossible to retro-fit this later.