Lessons from the workshop — the hard-won and the repeated.
Plan obsessively. Putting in the effort early to have a clear goal and the right kind of plan is usually the difference between success and failure.
Fundamentals are critical. Check your solder joints. Check your voltages. Document, especially decisions. Keep things clean and don't hurry.
Prototype → Project → Production is an order of magnitude more in difficulty and effort at each step.
Build to understand where it will fail. Where is the weak point?
After it breadboards, spend the time to set up the wires carefully. A lot of failures happen between components rather than inside them. Shortcuts to save time often end up wasting more time. There is test wiring, project wiring, and production wiring. They all look very different.
If you prototype with knockoff parts, they had better be a lot cheaper than the real stuff, because they're cheap for a reason. Buy two, learn what you need to learn, then go industrial quality.
Measure, don't guess. The meter, scope, and datasheet are usually more useful than another hour of speculation. It follows that you should first check the voltage. Always. Never assume. I hate when the magic smoke comes out.
Change one thing at a time. Otherwise you won't know what fixed the problem—or what caused it.
Leave margin. Don't design everything to the absolute limits of the specification.
Don't optimize too early. First make it work. Then make it reliable. Then make it elegant.
Know where the boundary is. Once you're crossing that boundary, things like isolation, guarding, fault detection, watchdogs, interlocks, EMC, enclosure, and grounding stop being optional.