Use tools that make it harder. Immutable datastructures, side effects constrained to a few places, good static typing etc. Where I worked before we had 100k LoC Elm projects with virtually no bugs. Now I'm working in a python/django codebase with probably thousands of small hidden bugs.
One thing that helps me a lot is: fail fast.
You program must break when something is not right because shitty lasts forever.
Immutable objects are great but you need to validate them when you construct them and throw an exception when something is not right. And functions should return what you would expect.
For example, this:
function GetEntitryById(int id):Entity
{
var entity = [action to get an entity];
if(entity == null) {
throw new Exception("Entity not found");
}
return entity;
}
function GetEntitryById(int id):Entity?
{
var entity = [action to get an entity];
return entity;
}