What aspects of the national government exemplify the framers plan to have representatives focused on national, rather than personal, interest?

I believe the answer is: short term office and the use of checks and balances  

Short term office refers to the maximum of time that a government representative had to held their position (they could be changed if the people are not satisfied with their works)
Due to the checks and balance system, other branches could monitor whether a certain government branch is acting within the scope of plan that they've create.