How might people organize a constitutional government to prevent the abuse of power

One major way in which people might organize a constitutional government to prevent the abuse of power is by setting up a system of "checks and balances" in which there is a "separation of power" (as is the case in the US government), since this prevents any one branch from becoming too powerful. 

Use the formula: (n-2)×180, where n is the number of sides(18-2)×180= 28800, now this is the full exterior angle of 18-gon so you divide it by 18.28800÷18=160