Developing web applications with ASP.NET and SQL Server involves creating complex systems that require careful planning, designing, and coding to ensure that the application performs well, is scalable, and is secure. Here are some best practices to consider when developing web applications with ASP.NET and SQL Server:

1. Use Parameterized Queries: Instead of writing SQL queries directly into your code, use parameterized queries, which are more secure and efficient. This can help to prevent SQL injection attacks and improve the performance of your application.
2. Use Stored Procedures: Stored procedures can improve the performance of your application by reducing network traffic, minimizing query parsing time, and making maintenance easier. Use them to encapsulate complex business logic and improve the scalability of your application.
3. Use Indexes: Indexes can speed up queries by allowing SQL Server to quickly locate the data it needs. However, be careful not to over-index, as it can slow down insert, update, and delete operations.
4. Implement Caching: Caching can improve the performance of your application by reducing the number of database calls required. Use caching for frequently accessed data, but be aware of the tradeoff between performance and data consistency.
5. Use Connection Pooling: Connection pooling allows multiple clients to share a single database connection, which can improve performance and reduce resource usage. Use it to improve the scalability of your application.
6. Implement Error Handling: Error handling is crucial for identifying and resolving issues in your application. Use try-catch blocks and custom error pages to handle errors gracefully and help maintain the security of your application.
7. Secure Data: Ensure that sensitive data is protected by using encryption, such as SSL or TDE, and implementing strong authentication and authorization mechanisms. This can help to prevent data breaches and maintain the integrity of your application.
8. Optimize Code: Optimize your code by minimizing the number of database calls, avoiding unnecessary processing, and optimizing queries. This can improve the performance of your application and reduce resource usage.
9. Test and Monitor: Test your application thoroughly and monitor performance and usage to identify issues and optimize performance. This can help to improve the quality and reliability of your application.

Developing web applications with ASP.NET and SQL Server can be a complex process, and it is essential to follow best practices to ensure the application’s performance, scalability, and security. By using parameterized queries, implementing stored procedures, using indexes and caching, utilizing connection pooling, implementing error handling, securing data, optimizing code, and testing and monitoring, developers can ensure that their web applications are secure, scalable, and performant. By incorporating these best practices into their development process, developers can create high-quality web applications that are reliable, efficient, and user-friendly.