It depends what my spies tell me. If there's a major production place for units i'll try to seige that while a second field built stack will try to intercept any hostile stacks. Basically rather than say seiging two cities i'll seige 1 and go hunting with the other stack.

I find Europe great for this with the mountain and forest choke points meaning your spies spot hostile stacks comming to the rescue.

Its pretty much the only way i've found of getting pleanty of field fights, AI seems to like just going for seige fights and bypassing my field armies (that have been squashing rebels ect)... maybe its just because of my positioning / garrisons / armies.