Generally they had two very different approaches. By ancient standards — not ours, of course — the Romans were stern but not sadistic conquerors.
Their standard tactic was to enroll defeated enemies as Roman allies or socii. The local elites (or at least, a biddable subset of them) would remain in charge of local affairs. They would be self-governing as far as domestic affairs went. The primary requirement was that the foreign policy of an allied state was firmly subordinated to Rome: no independent alliances or wars were allowed. Socii were required to contribute troops to Roman wars; these troops fought in independent units under their own officers, but high command was exclusively Roman.
The worst thing that usually befell a defeated enemy was the loss of some territory, which could be taken to provide land to Roman settlers who would live there in a new city of their own: a colonia. The colonia was in part a form of plunder, since it took valuable agricultural lands from the defeated enemy. It was also a military foothold intended to keep an eye on strategic locales. However coloniae usually worked as agents of Romanisation as well, particularly in places like Gaul and Spain where the local people would see a Roman colony as a valuable market, a source of exotic goods, and a conduit to the wider world.
Most conquered peoples were gradually assimilated into Roman citizenship. In Italy, this came about through an actual war: long time Roman allies fought to demand full citizenship in the Social War of 91–89BC. More often, local elites would become Roman citizens on a piecemeal basis. People farther down the social scale had fewer opportunities but it was hardly impossible: for example the apostle Paul, a Jew from the province of Cilicia in modern Turkey, was nevertheless a Roman citizen. Eventually the whole of a conquered region might acquire “Latin Rights,” a kind of limited citizenship for every free inhabitant.
The extension of citizenship completed the integration of all the upper classes across the Roman world: non-Romans eventually came to outnumber Italians in the civil service, the army, the Senate and in the ranks of emperors. Finally in 212 AD all free persons in the empire became Roman citizens — though by that time citizenship had little practical political meaning since the empire had no democratic institutions above the level of local government.
In general this system worked pretty well, and by the standards of the time it was fairly generous: the Romans only rarely resorted to the wholesale enslavement and depopulation of defeated enemies, which was otherwise not uncommon.
The flipside of this, however, is that Romans took a very grim view of “allies” who tried to reassert themselves. They regarded a surrender to themselves as a permanently binding contract, and they regarded any breach of that contract with unrestrained fury very different from their normal tactics. The most egregious violence that the Romans inflicted on defeated enemies — the sack of Syracuse (212 BC), the destruction of Carthage and Corinth (both in 146 BC), the levelling of Jerusalem in 70AD — was done to those the Romans regarded as faithless allies, rather than open enemies.
In short, the Romans offered their opponents a mix of incentives: good terms for easy surrender, but terrible punishment for what the Romans saw as “ingratitude” or “stubbornness”
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