Rules of Programming: Code For Performance
This is quite an important one. Code with performance in mind. This will probably save yourself some time and your customer both time and money.
However, don't optimize code too soon. It's pretty useful to optimize code afterwards, when you know the code is working.
Also worth keeping in mind: It can be smart to spend some serious bucks on better hardware instead of pushing man hours into optimizing code. The latter may come at a higher cost.
Most important thing though is to always code with performance in mind.
Rule #22 of Programming:
Code for performance