Always have your work reviewed by a peer / mentor / advisor before submitting for production.
Always do the reading. Make sure to read the datasheets and material for the hardware being used.
Know everything there is to know about the product or piece of hardware.
Be ready with everything that is needed to complete a job so as to avoid having to backtrack at the time the job is taking place.
Document everything!
Own up to your mistakes. Fix them. Learn from them. Be prepared with a solution for the mistake.
Separate the power supplies from the circuit with a resistor or fuse which can be removed during troubleshooting in order to identify where the problem may be coming from. This isn't always applicable but in some situations it can be very helpful.
Choose parts to be used in the design which can later be easily swapped out for alternative parts. Avoid unique parts which have a peculiar footprint or form factor.