They own the economy and manipulate it so they are always the winner. They establish racial boundaries and force their views and religion onto everyone.
Depending on how you interpret the first sentence, isn't this the essence of fascism?
Through some variation of romantic nationalism, every race is declared entitled to their respective home-system. The remaining stars are dealt out "fairly" by the Human Master Race (or maybe they're space-elves or whatever).
In sci-fi that takes place on an interstellar scale, the concepts of a home-system and homeworld are near-universal, so there probably won't be any claims to territories that aren't in the immediate vicinity of a star. However, depending on how FTL works, space-lanes or stargates might be claimed by someone. Perhaps by the Human Master Race, in order to consolidate imperial infrastructure, or by the races that have traditionally used them. In the latter case, expect the ruling party to call eminent domain.
On the other hand, there are occasionally two races that claim the same system as their home. If humans are one of them, the other is often from Mars, and is usually the first alien species they contact. If not, the two races are likely from the same planet, and have a symbiotic relationship (keep in mind there are 3 forms of symbiosis). It is also common for there to be at least one artificial race, whether robot, clone, or uplifted animal. (Though this probably goes without saying, their creators almost always consider them as property.) Regardless, the two races either fiercely hate each other or rabidly protect each other. Both of these are easily manipulated.
Who's to say the Emperor has to be the same person or even a single person? What if it was a group of people who ruled the empire and they spoke through the emperor who they control?
Like how in Equilibrium, Father has been dead for years, but his successors pretend to be him (for reasons that aren't clear to me). Or 1984, where the Inner Party speaks through Big Brother, in order to seem like they have a more coherent voice than they really do. Or in That Hideous Strength where NICE pretends to be run by a disembodied head, because no one would be stupid enough to join them if they knew they were really a Satanic cult. Or The Wizard of Oz where Oz pretends to be a giant flaming head so people will take him seriously. I should stop here, or this list isn't going to end anytime soon.
Also, I believe this link is relevant (keep in mind, the comments tend to be the most important part of this blog): High Kings and Galactic Emperors.