Writing code might seem easy when you start from scratch, everything is simple and obvius. Unfortunately, things get messy preaty quickly if your are not carefull with how you organize and write your code. Applaying some or all of the SOLID principles can help you prevent the mess.

SOLID is actually an acronym for five design principles which I list and interpret below:

S: Single-responsibility principle

A class should only have a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class.

O: Open–closed principle

Software elements should be open for extension, but closed for modification.

L: Liskov substitution principle

Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

I: Interface segregation principle

Many client-specific interfaces are better than one general-purpose interface.

D: Dependency inversion principle

One should "depend upon abstractions, not concretions.