I organize my stacks in what I call Combined Armed Army Groups. Generally, 9xMelee (predominantly CR), 9xSiege, 4xMounted, 4xStack Defenders (Drill Promos), and a Medic. I just keep my Gunpumps producing more stacks and replacing losses, while my other cities produce garrison troops. For the mid-game post-Gunpowder push, I'll try to start with 4 to 6 such groups. By the end of the game, about 15 is fairly normal, although I've had 25+ on a couple of occassions (I play Huge/Marathon, btw). The most important thing is the Siege - if you have enough Siege all you need after that is mop-up troops. Attacking on multiple fronts with multiple mutually-supporting stacks gives the AI fits; it will generally counterattack all it's troops on a single front, after that it's a cake walk.
Oh, and try to bring along a Scout / Explorer or two with each stack. They're cheap and expendable and are great for probing ahead of an advance and exposing the AI stack before it can counterattack, which gives you the initiative to smash it with your Siege first.