Try to use as few different parts as possible. Try not to use parts that can be confused easily, such as resistors with the same value but different wattage or tolerance. Use good parts unless the cheap parts cost significantly less. The engineering and repair time saved with the better parts is significant, too. This is a schematic that contains elements that are common to many designs and will be your starting point for all future designs.
Test points are handy.
On ground nets, make them large enough to easily connect test equipment ground leads. Make extender cards to allow trouble-shooting if your PCB's are closely spaced. Commit all spare inputs of logic gates or op amps. Use keyed connectors. Try to make every connector different so that they can't be confused.