Proper module naming and structuring:

Module names should be descriptive yet concise to indicate its purpose at a glance. Avoid generic names like “Module1”.
Group related modules together in a logical folder structure for easy navigation. Common structures include grouping by functionality, data types, projects etc.
Each module should focus on performing a single well-defined task. Splitting large modules into smaller focused ones improves management.

Use consistent code formatting:

Adopt a consistent indentation and whitespace usage to improve readability. Maintain a blank line between sections for visual separation.
Follow a logical consistent order to define and call subroutines, functions and variables. Common orders are alphabetical, chronological or functional grouping.
Add comments liberally to explain the purpose, inputs, outputs, limitations of sections of code. Well commented code is easier for others to grasp.

Avoid direct workbook/worksheet dependencies:

Hardcoding worksheet or workbook references should be avoided as it reduces reusability of the module across workbooks.
Use variables to refer to worksheets, cells or ranges to make the module portable. Provide parameters or functions to initialize these variables.

Parameterize inputs and outputs:

Define and pass required and optional parameters to subroutines/functions rather than using hardcoded values within them. This improves reusability and testing.
Return values using parameters passed by reference rather than directly modifying sheet cells or ranges from within the module.
Provide parameter validation and error handling for incorrect or missing parameters.

Implement error handling:

Anticipate potential errors and add On Error statements with descriptive error messages. Use error codes rather than generic “error” messages.
Handle common runtime errors gracefully rather than halting code execution. Log errors and continue processing where possible.
For non-critical background macros, enable error handling and resume next rather than stopping processing on errors.

Encapsulate logic in reusable functions:

Identify blocks of repeated logic and extract them out into well-named reusable functions with related parameters.
Functions should perform one logically related task and return a value rather than modifying sheets.
Functions make code modular, readable and easier to debug, modify and test in isolation.

Use constants and naming:

Declare constants for fixed values used in multiple places like column numbers, error codes etc to avoid hardcoding them repeatedly. Self-documenting names are used.
Give variables, cells and ranges meaningful names describing purpose/content rather than generic names like myVar, CellA1 etc. Use camelCase, underscores or PascalCase as per conventions.

Follow best practices for VBA coding:

Implement standard OOP principles like encapsulation, loose coupling, inheritance where relevant for object-oriented modules.
Add relevant help documentation for public interfaces using syntax like “VB_Header” and “VB_Help”. Inbuilt IntelliSense can then surface this.
For shared use, digitally sign and compile modules as add-ins for distribution. Provide uninstallation support.
Consider using optional structures for configuration that can be initialized based on workbook/user specific needs.
Follow standard code formatting, commenting and design principles as per industry best practices.

Implement testing:

Gradually build a comprehensive testing suite to validate functionality, catch regressions as code evolves.
parameterize tests using constants or shared variables for flexible maintenance.
Automate execution of full test suite on regular basis as part of continuous integration/deployment workflow.
Test boundary conditions, exceptional cases, performance in addition to regular validation scenarios.

Document the API/user guide:

Along with code comments, provide an overall technical documentation for module listing purpose, prerequisites, public interfaces, examples, limitations, frequently asked questions.
Consider online documentation or help files in add-in for end users in addition to in-code comments targeted for other developers.

Adopting these best practices while structuring Excel VBA modules helps create well-designed, organized, readable and maintainable code base which is more resilient to changes, easier to understand, extend and reuse in future. Proper planning and modularization pays off especially for large, complex and mission-critical deployments involving a team.