A human player can often ruthlessly exploit AI allies to siege for him, if he has a leader. Get an AI with a decent army into alliance. Ideally it will not be a major power, since they tend to have leaders of their own. (You can mouse over a potential ally's armies to see if there are any leaders currently; you can also look at their leader file.) Now you declare war on a third party, and invite your ally to the war. The ally will send his armies to siege an enemy province or two; you should concentrate on covering and sieging the enemy's other provinces, avoiding all battles except for sure wins. Eventually the enemy will blunder into your ally. Once your ally gets a siege into the "yellow" zone, where there begins to be a chance of taking the city, you move in your leader and take control. Once you get to land technology 5, you may want to assault; in this case, follow the leader in with a small infantry unit to kick off the assault.
On one hand, it would make the most sense to keep a siege thief army as small as possible, preferably with cheap infantry. On the other hand, too small of an army can result in you accidentallying killing your valuable leader. They are scarce, generally last for about 20 years and it would be nice if they stuck around to steal again. Probably the best size is five to ten thousand men. Some attrition is acceptable as the stolen provinces are worth it, even in the ridiculous dogpile sieges the AI does. The AI steals sieges too, sometimes in the same surprise manner, even if it isn't deliberate about it.