Build the engine with about 8.5:1 CR (Compression Ratio). More CR needs more octane.
Use a four-inch or smaller bore. (Bigger bores require more octane)
Use aluminum cylinder heads if possible. (Fewer hot spots with better heat transfer)
Use a 180 degree thermostat, a good water pump, efficient radiator and effective fan system (High temperatures need more octane)
Use an aluminum intake manifold. (These improve cold start driveability)
Be sure air / fuel mixture is correct. (Lean mixtures require more octane)
Cool inlet air is a benefit. Use a fresh air system if possible. (A 25 degree increase in inlet air temperaturerequires 1 more octane number)
Live at a higher elevation. This may not be practical, but for every 1000 feet above sea level, your engine needs 1 less octane number, but makes less power.
Live in a humid region. Again, this may not be practical, but if you live in Florida and the temperature is 85 degrees, your car will be satisfied with a gasoline that is 2, or 3 octane numbers lower than it would if you were in Phoenix at the same 85 degrees temperature. This is why water injection works in the southwest U.S.
Reducing spark timing by 2 degrees will reduced the engine octane appetitie by 1 octane number. 4 degree is the maximum for this "fix".