To help things along where I can, I train these allied units in advance.

For example, most area's share common troops, so as Carthage in southern Iberia, as soon as I moved out to conquer Iberia, I took with me several well trained(upgraded) units of Milites, so that after capturing a settlement, even before installing my Client Ruler, it's correctly garrisoned, and I can move straight on to the next objective.

I did this in North Africa, Italy, and Greece, and it's cheaper than mercs as the first city captured is the mother to all. If you don't share in advance, any common troop types to make in advance, that is!