The biggest way to conserve ammo is to make sure you don't use camping strategies and run trains of zombies instead. Let zombies build up to large groups so that when you fire into them, you're hitting 4-10 times more zombies per bullet, reducing the amount of ammo you'll expend. If someone's got their own group of zombies, don't waste your ammo on it as you and they are going to end up burning twice the amount of ammo for the same amount of zombies. In later rounds, use nukes when you see them and zombies have built up well. Also, always have a wall weapon as a primary weapon so that you don't have to worry about not having ANY ammo at all.