1. Plan the application architecture and design with security in mind from the start. Consider things like authentication, authorization, input validation, etc.
2. Implement secure coding best practices to prevent vulnerabilities like XSS, SQL injection, CSRF. Validate, sanitize and encrypt all inputs.
3. Use a framework like Django or Ruby on Rails that incorporates security features and keeps applications updated.
4. Configure infrastructure securely following the principle of least privilege. Use WAFs, DDoS protection, secure protocols, and monitor for threats.
5. Test security at all stages of development using tools like ZAP and Burp Suite.