how would you go about making an oppressive system of government?

imagine if you will that you are an elf that is brutally racist to the humans you rule over in order to keep your blood pure or whatever. You and your race rule over humans in segregated cities yet humans outnumber you, and are probably the source of the majority of your creature comfort*. E*ventually, to meet your demands, certain humans will step up and eventually will get very wealth. Wealth is the key to power, but you foresee this. Aside from putting it into law than no human can hold any position of political significance, what else can you do to make even rich humans politically impotent and have it last?