As Seedexodus says, the key to this is to park full stacks around the enemy faction's cities (optimally more than one stack on more than one city). This is seen as a threat by the AI, who will be much more likely to give you a Ceasefire, since the AI faction will gain an immediate end to the threat you've got sitting on their doorstep(s).

If you conquer a province, then your ability to threaten the enemy actually *decreases* according to how the AI thinks of things. So keeping your full stacks visible and positioned well is important.