This is just for internal factors of development. Think like you're a leader of a country.
Another good example of a long lasting empire would be that of the Ottoman Turks and unlike the other empires discussed in previous posts during it`s height it never tried to impose one culture or even religion on it`s subject peoples like the Greeks ,Serbs and Armenians .It was only in it`s later years as it`s power declined that they began to attempt to impose one way of doing things on it`s subject peoples .So if you want a long lasting empire made up of different races and languages they would be a good example .Just don`t let feelings of racial and ethnic unity destroy what you have built three hundred years down the line .Hence the Ottomans were a good example of what to do ,and what not to do at the same time .
Does Imperial Japan count?
Economics. Spain had all the national unity you could want, but underdeveloped middle classes.