One mistake that a lot of players make when they start transitioning to the semi-pro and pro-level is to give in to the fish-out-of-water feeling that accompanies sitting down at a Grand Prix for the first time. Whether Round Four comes along and you find yourself matched up against a future Hall of Famer, or whether the first hand you draw for the tournament is six lands and a five-drop, whatever the situation, you have to clear your mind and, calm down, and focus on what you need to do.
Execute your gameplan. Of course, that means you have to have a gameplan, so if you don't, well, chalk that up to mistake number two. It is absolutely amazing how few people can articulate what it is that their deck is supposed to do. As an example, let's look at what happens when someone picks up Red Deck Wins for a PTQ.
They might have a short answer to the question about their gameplan, something like, "My goal is to deal 20 damage as fast as possible with creatures and burn spells." Even that limited plan is better than nothing, but if you really want to be playing the deck as well as it can be played, you should be able to explain to your friends and teammates that your plan is to
1) Apply early pressure with a combination of one and two drop creatures.
2) Control the board using direct damage and by economically trading for value in combat.
3) Use more powerful cards to craft an end-game involving a flurry of direct damage, or a massive
swing with powered up creatures.
If it sounds like common sense, it is, and this tip isn't meant for you, but you would be surprised by how often this minor detail gets overlooked. Mike Flores has written several articles about pre-visualization and the importance of your gameplan, and it never hurts to review the foundations that the rest of your game is built on.