Well, my No. 1 is to always play tight. From my experience, most new players play about 80% of the hands they are dealt in Texas Hold'em. In fact, the reverse should be the case; they should only play about 20% of the hands they are dealt. This probably isn't earth-shattering news to anyone who has played a reasonable amount of poker. But it remains a concept that is worth understanding through and through.
The concept of playing tight actually only applies to the overall percentage of hands you play. When you are first to act or under the gun you should play so tight that you only play around 5% of the hands you are dealt, while in other positions, like the button or the big blind, you can drastically expand that percentage. It is only the average that should be around 20%. Up front almost no hand should look good to you unless it is AQ or better or at least 66 or better. But why so tight?
There are many reasons to play super tight up front but the one I want to focus on has to do with a decision making disadvantage. When we play poker we never want to lose sight of the fact that this is a game of decision making. You have a decision at each point during a hand when the action is on you and if you are better at making those decisions than your opponents you will win lots of money. The road to becoming the better decision maker is to bring to bear the maximum amount of information available to you. In poker, this means using the information available to you to narrow down the holdings of your opponents. And therein lies the problem with playing loose up front. You will always be acting with the least information available because you don't know what your opponents are going to do after you. They will always know what your action is when they have to act. They will have been able to watch you look at your cards. They will have been able to study your face, your body posture, and the way you threw the chips in the pot when you acted. You? You will have none of that information.
Consider an example from the business world. In business negotiations it is a huge disadvantage to go first. When you go first you have no idea what the other person's range might be so you run the risk of negotiating against yourself. It is a disaster when you offer an amount in a negotiation that is higher than the amount the other negotiator would have accepted. But it is difficult to know ahead of time what the other person's range will be.
Therefore, if you are to act first in a negotiation it is better to act from a position of strength. You need to have done the maximum research on the topic, have acquired the most information to offer the most reasonable number one that is lower than what your opponent wants but close to his range. You need to really have put a lot of time and effort into coming up with an offer before the negotiation.
The person second to act in the negotiation? Well they don't have to work quite as hard because the mere act of the other person offering first puts them at a huge advantage. They already know the other person's range and can then negotiate based on this information. That is not to say that the person second to act doesn't need to know their stuff. They do!! It is important that they know if the first person has lowballed them, but they don't need to be quite as strong acting.
Poker is just like this. The person acting second can really loosen up. They can call raises with hands they would never raise in first position with because they know they will be acting second in the rest of the decision making process, acting with the maximum amount of information. They will be able to limp in with a much wider range of hands when people have limped before them because the dangers of getting raised behind you are so small when only the blinds are left to act. And when the action gets to the button and everyone has folded in front of them, they can raise with 70% to 80% of the hands they are dealt, partially because they know they will act last for the rest of the hand.
The poor guy under the gun? Well for him the opposite is always true. He can't limp in because the risk is too high - being raised behind him, having no idea of what the 9 people left to act are going to do. For the rest of the hand he knows he will have to act early in the decision making process. Having no idea of the quality of his 9 opponents' holdings, he has to be much more aware of the hands he raises with.
Being the first to act in poker is like being handcuffed to the decision making process. So play tight up front.