I have 2 that have stuck with me for 20 years...
#1 - Take the first available shot. If a deer is broadside at 30 and you're comfortable...shoot. You don't get too many opportunities.
#2 - Sit the hell still. I see way to many hunters that look around like they are expecting a stampede. Many deer that come in may have seen a movement way before they get to you and know where to look...even if they don't know what they are looking at.