[Taqwus]

You don't, really.

It's quite deliberate; it's a time of war between nations, not one of peaceful prosperity. At best you take Growth, and perhaps Luck and Order (these two to minimize the risk of pop-killing events; some of these are quite bad -- killing 25% pop, say --- and at least one is extremely bad pop-wise but rare (Ancient Presence event)). You also need to avoid overtaxing (people leave, plus unrest), or causing more unrest than you can afford to deal with by lowering taxes, and go easy on the blood hunting.

If you're desperate, you could Wish for population.

If you want to increase the effect of growth/death, you could mod it.