In the long term, poker is no doubt a game of skill.  I don't just mean it's mostly skill over the long term, I mean poker is 100% skill in the long term.  If you're an unskilled player you will definitely be a loser in the long term.  If you are a skilled player, you will be a winner in the long term.  The only time luck has an effect is in the short term.  These short term swings are what keep bad players coming back to the table time and again.

One of the most difficult things for poker players to do is separate themselves from the results of the decisions they make.  If you make the correct play, the outcome of the hand doesn’t matter one little bit.  The only thing that counts in poker (in the long run) is making the correct decision.

It's easy to go on tilt and lose more money than you should if you don't remember this concept when you take a bad beat.  If you make the right decision but your opponent wins the pot, don't feel bad about it.  Just know that as long as you continue to make the correct decision, you will consistently make money in poker.

This is easier said than done, though.  One of the areas I struggle most in is avoiding becoming weak tight during bad runs of cards.  I know I should almost always 3-bet with AK preflop but if I've been burned the last 4 times, I get tempted to just throw it away and be done with it.  Seeing monsters under the bed is a very common but very subtle form of tilt.

Oh and also, don't inform the other players at the table that you made the right decision but the other fool just got lucky.  Nobody else cares so keep it to yourself.  There's no point in broadcasting how great you are to the table.  The only thing that will accomplish is killing your action as the other players realize you actually know what you're doing.  Let them think you're a losing poker player.  It's better that way.

This also goes the other way.  If you make a bad decision but get lucky and win the pot, make a mental note of what you did wrong.  If you just happily scoop the pot and move on, you're not going to learn anything.  Being able to learn from your mistakes (while still getting paid for it) is rare in poker so don't waste the opportunity by forgetting the hand.  Analyze it and see what you could have done differently.  It doesn't matter how lucky you are.  If you consistently make bad decisions you will lose money in the long run.

Getting lucky and winning a pot is more dangerous for new players than just playing bad and losing.  If a new player hits a few hands and goes on a big rush, he's naturally going to think he's more skilled than he really is.  When reality kicks in it's even more painful because that player is going to lose even more money.  This is especially true if the unskilled player mistakes his earlier luck for skill and thinks he just got unlucky when he lost all his money.  It can lead to a skewed perception and more deposits than that person originally intended to make.  Make sure you never fall into this trap.  Honesty pays.