[Dedas]

Expect the unexpected.
Read on the forums that building more than 1 in PD is a waste of money? Do not get surprised if the enemy takes all your provinces at once with small stealth armies.

Always have a plan B.
What will happen if you do not win that big battle? See to it that you know and that you have a backup plan.

Do not put all your eggs in one basket.
One big army is as vulnerable to your miscalculations as two smaller armies but at least you get a second chance when those bombard rituals starts falling.

Know your enemy.
Use scouts.
Use diplomacy so that through other players you will get knowledge of how your enemy fights and if he is trying to betray you soon.

And so on...