What are the suggestions or a proposition for a developer to take care when writing a code or developing an application from scratch?

When we start developing an application from scratch, things are pretty straightforward. However, when continuing developing and the business logic becomes harder and harder we struggle a problem of performance and many other issues. Sometimes, we have to write 'dirty code' or not writing well in order to achieve our goal and accomplish the task. The probelm is when your are a freelancer and you don't work in a team with seniors and architects you may don't take care about some details and write code that could be dirty and you don't respect the best practices or performance or another application limit. What are the suggestions for a freelancer to make good application without the guidance of seniors and architects?