Some things to keep in mind.
Rating is in no way a reflection of how good a player is. Some people are amazing deck builders, but suck at playing. Then you have the people who have no clue of how to build a deck, but are amazing players.
The trick is to find out what YOU are best at.
As Magnechu said, KNOW YOUR DECK LIST!!! I can't even begin to tell you how huge it is to know every card in your deck. Play test a lot and it really just becomes something you memorize with out even really trying.
Here's a really simple one. Don't misplay, and always watch for your opponent to make a mistake so you can take advantage of it. I almost never misplay (except when I'm really tired or distracted
) and it REALLY helps you do better. In close games a singe misplay can decide it, and who ever can take advantage of it usually can cause some serious damage.
Use smart deck choices. Really, there is only so much you can do to prepare your deck for a tourney. But what it really gets down to is make it CONSISTENT! Don't over tech (youll never be able to tech for everything so dont even try), only 1 or 2 small tech lines should be used. Make sure you pick a deck with good/even matchups against the decks in your area. And the really big one.... DO NOT USE LUCK BASED DECKS. This is a HUGE mistake, as luck has a tendency to fail you when you most need it. While luck based decks can be very good, they are inconsistent.
Don't be afraid to use the BDIF because you think otheres will think badly of you. If a deck is BDIF, its BDIF for a good reason. Not using it because you want to be different, or just beacause you don't like it is plain stupid. Its increadibly hard to build a good red face paint deck that consistently wins. Sometimes simple is best.
NEVER underestimate a deck or player, expect EVERY match to be hard, and every player to be good. Underestimating a player/deck can be a huge mistake, that can eventually lead to you losing.
The last thing I will mention is not only know your deck list, but know your deck. Learn how it works, what to do if you get a bad start (yes all decks get bad starts so its good to know how to play out of them), learn every aspect of how your deck can function untill you know it by heart. I've won battles by attacking with Claydols. Many people would never even look at Claydol as an attacker, but at times he can be the perfect attacker. Being flexible is undoubtedly the most importent part of playing with your deck. You have to be able to change strategy at an instant (a good idea is just expect someone to ruin your perfect plan as it usually happens anyway ;p). Above all, dont over think it. If you think to hard about a given battle, it can mess with you, and you can actually misplay due to forgetting something.
Theres sooooooooooo much more you can do to improve yourself as a player. A lot of it is just stuff youll figure out on your owne.