I have found Akin's Laws of Spacecraft Design to be very applicable to the design of not only spacecraft but medical devices and other mission critical products as well. My favorite are the first six:
1. Engineering is done with numbers. Analysis without numbers is only an opinion.
2. To design a spacecraft right takes an infinite amount of effort. This is why it's a good idea to design them to operate when some things are wrong.
3. Design is an iterative process. The necessary number of iterations is one more than the number you have already done. This is true at any point in time.
4. Your best design efforts will inevitably wind up being useless in the final design. Learn to live with the disappointment.
5.(Miller's Law) Three points determine a curve
6. (Mar's Law) Everything is linear if plotted log-log with a fat magic marker.
and a few of my other favorites:
8. In nature, the optimum is almost always in the middle somewhere. Distrust assertions that the optimum is at an extreme point.
9. Not having all the information you need is never a satisfactory excuse for not starting the analysis.
10. When in doubt, estimate. In an emergency, guess. But be sure to go back and clean up the mess when the real numbers come along.
13. Design is based on requirements. There's no justification for designing something one bit "better" than the requirements dictate.
14. (Edison's Law) "Better" is the enemy of "good".
15. (Shea's Law) The ability to improve a design occurs primarily at the interfaces. This is also the prime location for screwing it up.
20. A bad design with a good presentation is doomed eventually. A good design with a bad presentation is doomed immediately.
34. (Roosevelt's Law of Task Planning) Do what you can, where you are, with what you have.
40. (McBryan's Law) You can't make it better until you make it work.
42. Space is a completely unforgiving environment. If you screw up the engineering, somebody dies (and there's no partial credit because most of the analysis was right...)
Here is the complete list of Laws: