RULES OF SOFTWARE ENGINEERING

CLARITY BEFORE ANYTHING ELSE!
Explain what you are doing by adding comments, choose clear names for files, classes,  variables, functions and whatever. Use indentation properly and break lines that are too long and complex.

USE CODING STANDARDS!
Computational software is always teamwork. And not only that: it is often created over generations of developers. Unless you agree on a coding standards, the code will quickly turn into a thrift shop of styles.

AVOID TECHNICAL DEBT!
You've probably experienced it: working on a code that is old fashioned, badly programmed, unintelligible or wrong. This debt may get so high that your code ends up a total loss. So improve the code that needs to be improved.

USE THE RIGHT TOOLS!
These days, a version management tool, a continuous integration tool and an issue tracker are standard tools for all software engineers. Good tools are so readily available that there is no valid reason not to use them.

DONT OPTIMIZE UNTIL NECESSARY!
Optimizing is usually a matter of rearranging your code to make better use of the hardware & time. Write the code as it should be from an algorithmic and code quality point of view. Then when necessary you can optimize your code.

DO THE RIGHT DOCUMENTATION!
Future developers will need some basic documentation to find their way around the code. Make sure that the code is internally documented because that is where developers will look once they've understood the basic plan.

USE OPEN SOURCE LIBRARIES
Most of the basic algorithm are available for free in excellent numerical libraries. Unless you've come up with with a fundamentally new approach, you would be wasting your time and money.