Mastering Ruby on Rails development can dramatically enhance your web application skills. Here are 5 essential tips to help you along your journey:

1. Understand the MVC Architecture: Familiarize yourself with the Model-View-Controller pattern, which is at the core of Rails. This will help you organize your code effectively and understand how data flows through your application.
2. Leverage the Rails Command Line: The Rails command line allows you to generate scaffolding, run migrations, and perform other functions rapidly. Utilizing commands such as rails generate can speed up your development process significantly.

In addition to the foundational knowledge, continuous learning is key. Consider these final three tips:

3. Utilize Gems Wisely: Gems are libraries that can add functionality to your application. Learn to choose and implement gems that best fit your project requirements without overcomplicating your codebase.
4. Test-Driven Development (TDD): Always aim to write tests for your code. This will not only help you maintain high-quality code but also improve your debugging skills.
5. Join the Community: Engage with the vibrant Ruby on Rails community through forums, conferences, and online discussions. Networking with fellow developers can provide invaluable insights and assistance.

When developing with Ruby on Rails, one of the common pitfalls is neglecting the organization of code within the application. Many developers tend to place too much logic in controllers, which leads to fat controllers and difficulties in maintaining the code. To avoid this, it's essential to follow the Rails convention of keeping controllers slim and delegating complex logic to models or service objects. By doing so, you enhance the readability and reusability of your code, making it easier to manage as your application grows.

Another frequent issue is inadequate database indexing, which can severely impact application performance. Without proper indexing, queries may take much longer to execute, especially with larger datasets. This can result in a sluggish user experience, which is detrimental to user retention. To prevent this, always analyze your database queries and add indexes to columns that are frequently searched or used in JOIN operations. Additionally, regularly use tools like Bullet to identify N+1 queries and optimize your database interactions accordingly.

Building scalable applications with Ruby on Rails requires a solid understanding of both the framework and the best practices that support scalability. Begin by setting up your development environment. Ensure that you have the latest version of Ruby and Rails installed. Once your environment is ready, create a new Rails application using the command rails new your_app_name. This foundational step is crucial as it allows you to establish a clean and optimized structure for your application. Next, consider utilizing a service-oriented architecture, which can help in distributing loads across multiple services, enhancing scalability.

After the initial setup, focus on optimizing your application for performance. You can achieve this through various techniques, such as:

1. Database Optimization: Use indexing, eager loading, and caching strategies to enhance your database interactions.
2. Code Optimization: Regularly refactor your code to improve maintainability and efficiency.
3. Caching: Implement caching mechanisms like memcached or Redis to reduce database load and increase response times.

By following these practices and keeping scalability in mind from the very beginning, you will be well on your way to building a robust, high-performance application with Ruby on Rails.