Any electrical system that may be unsafe to approach while powered should have a manual kill switch installed that:
Disables all energetics or potentially unsafe components.
Can be operated without requiring the rocket to be taken off the tower (e.g. mag-switches).
Does not require any software to work.
Wiring should be done so that the circuit is as readable as possible. If supplies allow, use a different insulation color for each wire. At the very least, positive should be red, ground should be black, and all other wires should not be red or black.
If there is space, and it is likely wires could get easily mixed up, wires should be labeled.
All switches and connectors need unambiguous labeling. Labeling should be permanent (i.e. not on tape, etc).
All wires used should be rated for the current going through them.
Strain relief:
Wires that get handled, moved, or torqued at any point should be strain relieved on both sides of that point.
Wires should be relieved to the point where they can survive very violent shaking (especially if the system will be going in the rocket).
Avoid tight wires between two points.
RC lithium polymer batteries should be fused as they can sink up to 50 A.
When designing a box, circuit layout, or other assembly, keep in mind that wires and electrical components do take up space. Leave clearance in the design stage; this problem should be solved before fabrication/assembly.
If possible, put hot-swappable components such as batteries or modular PCBs in locations where they are easily accessible.
Use standoffs to make sure the circuit board has good isolation from what’s beneath it. This is especially important if the board is mounted on a metal plate.
When using shielded cable, ensure the shield is properly terminated and grounded.
If it’s conceivably possible for a component to be installed backwards, there should be idiot proof documentation of which way it goes, that documentation should be less than 2 inches from the install point.