Thou shalt not underestimate Assault Terminators, for thine arse shall be raept by their Thunder Hammers
Thou shalt not assume that one rapid firing squad in thine army is enough.
Thou shalt not underestimate the Rune Priest's power to cancel thy own.
Thou shalt not underestimate Logan Grimnar, for he is asskickery incarnate.
Thou shalt not leave home without Obliterators, for even though they reeketh of cheddar, they areth the very best thine Heavy Support haveth to offer.
Thou shalt not look down upon Gift of Chaos, for there is nothing like a Necron Lord on a Destroyer Body becoming a pile of violent goo. (I speaketh from experience)
Thou shalt not assume that the Leman Russes are the greatest threat in the IG army. Their heavy weapons and command squads are key.
Thou shalt not assume that you need only kill a squad of Gaunts once, for they are Without Number; consequently, your Heavy Bolter and Demolisher Cannon rounds must be without number also.
Thou shalt not become a crybaby when the Gods turn their gaze away from you and thou rolleth ones like they areth earning thou money. Thou shalt acccept thine defeat and implore the Gods that they may go easy on thine arse next time.
I guess thats all the wisdom I can think of right now. I wish I knew all this before I learned them the hard way.