1. Know your ArmyBe able to remember the basic stats of your guys without having to check the book every five minutes. Be familiar with the types of units you struggle with. Be familiar with the more common rules interactions that may come up. Remember their special rules! Forgetting you have feel no pain on your warlord not only may cost you the game, but you will be kicking yourself for it a really long time.
2. Play More GamesEven if your personal win/loss ratio is 0/20, you are still miles ahead of the player that has only played four or five games. Penalize yourself for making mistakes. If every time you forget to charge your orks until three different combats have resolved your opponent lets you charge anyways, you will keep making that mistake. Then come tourney time you forget and the opponent says no. You may lose that game just for forgetting something!
3. Read the Event PacketMost events will tell you which missions you will be playing. You should then ask yourself a few questions:
How many kill point missions vs how many objective missions are there?
If there seems to be a lot of kill points missions, you should probably take big, tough units that are hard to remove in order to deny kill points and it is best to leave easy to kill transports such as raiders or razorbacks. If there are a lot of objective missions then mobility is your friend, so bringing fast transports give you a huge edge.
How many points are you playing?
In low point games, big nasty vehicles will be powerful because anti-tank is at a premium due to generally high point cost. Making sacrifices to bring multiple anti-tank threats will be well worth it.
Larger games make specialist units shine, you have enough points to field them and they will be able to do their jobs.
What's the meta like?
If you know that there will be a lot of one type of army, you will need to bring things to protect yourself. Bringing skyfire when everyone that is going to the tournament is taking Eldar/Dark Eldar/Tyranids becomes a really good idea.
4. Build a Take-All-Comers ListWhat is your army weak against? AP3 weapons? Big close combat squads? Giving your list interlocking redundancy that will solve these issues. A lascannon devastator squad will perform well against monstrous creatures as well as vehicles, and having sternguard drop in with combi-meltas also protects you in case you lose one squad or the other early on. Taking units that can do well against multiple treat types is at the core of take all comers list building. During the course of normal games, never list tailor against the army you are playing against. Tailored lists DO NOT WORK at tournaments. This is a crutch that you should put away immediately, it will make you weak.
5. AttitudeThis is the most important tip I can try to give you. It doesn't matter if you look at your opponent's army and see a net list calculated to destroy. That DOES NOT mean that the game is already lost. It is entirely possible that that dude has no idea what he is doing and just took what the internet perceives as being over powered. You will win against this player if you know exactly what to do with your units. You should approach every game with a winning mentality. Even if you lose, you were trying to win every turn of the game, and because you payed attention you will probably know exactly why the game went south so that you can avoid it happening again.
Hopefully these tips help you out, and they are basic things that a lot of people with they knew when they were starting. Don't let your inexperience stop you from attending an event, if you walk out with a 1/4 win/loss ratio I guarantee you learned a ton of lessons and gained valuable experience that you will bring to every game you play after that. Get in there and mix it up!