UT2004 is far more complex than many would give credit for. I will try to explain some of what I have learned over the years.
Time is the only way to get good at UT2003/2004. Play the game a lot. Fighting bots helps, but do not expect it to turn you into a master. Play a good share of the various modes. Free for all deathmatches are probably the best way to improve your skills. It is necessary to understand that in UT2004, there are two categories of your skill. Offensive and defensive. Offensive skill is obviously your ability to attack the opponent, while defensive skill is obviously your ability to survive. You need both to be able to handle the more difficult opponents that you may face.
The most common type of player is one with a good offensive skill and little to none on the defensive side. Examples include players who stand still and keep shooting shockrifle combos at you until you die, or players who have impressive aim with a few weapons and just shoot at you relentlessly. Think; Deathmatch to 25 kills, and the leading guy has 25 kills and 25 or more deaths. A much less common type of player is one who has a high defensive skill and low attack skill. These players usually have a very small amount of deaths in a game and also a very low level of kills comparatively. A good player has lots of skill and knowledge in both. For example, in the games I played with Fatal1ty(One of the higher skilled players) he would usually win with 75 kills before opponents had even 20 and have under 5 deaths. That shows that he can both survive attacks well and dish out quick and effective attacks.
Learn how each weapon works and when the right time to use each occurs. Every single weapon has strengths and weaknesses and the proper place to be used. Do not just get good with the rocket launcher or minigun and be satisfied with that like so many players do. Learn to be able to kill all of your opponents quickly with any weapon, including the shield gun and bio rifle. When you have multiple weapons, never use just one in combat. A great technique that is used by the more skilled players(Fatal1ty for one) is to have the lightning gun out 24/7, then when you see an enemy, get a quick shot on them with it,(This shot will usually do 70+ damage) and then switch to a quick attack(instant or near instant) weapon such as the shock rifle or minigun and finish off their last little bit of health with that. Most weapons are not instant shots, as a result of this, you must learn and know the time it takes for your projectiles to reach the enemy. Also, chances are, your enemy will not be in the same place when the projectiles land as when you first shot them. Learn enemy movement. To become skillful and effective, it is necessary to learn how to predict where the enemy will be. Most players dodge to a certain pattern, because of this, you can quickly and effectively know where they are going to dodge to when they see you fire a rocket or another weapon. Shooting at where the enemy will be is usually more effective than shooting at where the enemy currently is.
Defensive skill, contrary to popular belief, is far more complex to achieve, and can only be achieved effectively if you already have a good level of knowledge on how the weapons, jumping mechanics, and physics in general work in the game. ALWAYS pick up health vials and adrenaline when you see them, even if they are out of your way. Always pick up energy shield and extra ammo and weapons(Even if it is a weapon you do not much care for). Try to move in a circuit around the map. Always arive at the location of health vials, adrenaline, energy shields, and double damage just as they appear. Doing this correctly, you can obtain a monopoly or so to speak of the map. You will end up always having more health than your opponent and usually more ammo as well. Never underestimate the shield gun. If an enemy shows up out of nowhere with double damage and a minigun, you will usually have less than two seconds to decide the battle. Getting out the shield gun and using the shield can give you the time necessary to jump behind a corner and whip out a proper weapon that could get rid of the opponent in one blast. Take time to learn what I mentioned above. 99% of players do not do any of what I just said, and thusly, they usually get their ends handed to them playing against a player who knows how to do what I have been saying. Next up that needs to be learned is effective dodging. Dodging, as you know, is simply avoiding weapon fire. However, it is a bit more complex than what one would think at first glance. When you dodge, the opponent has to adjust their aimer accordingly to counter your dodge. When you dodge, you want to get to a location that will not only be out of the blast radius of whatever weapon is being fired at you, but you also want to be able to successfully continue dodging. Many people I play against will dodge my first few rockets, only to lead themselves into a corner, and thusly, their demise. Movement is also another skill that is necessary to be learned if you want to be effective. Learn to move faster than everyone else. Dodge jump forward and then jump again before landing. This type of movement is radically faster than ordinary running and will allow you to put considerable distance between(Or dramatically reduce the distance) you and your opponent. Always place yourself in the hardest place to be hit. The shield gun also has the impressive ability to completely reverse the direction of some projectiles going for it. While I have not mastered it yet, there are those who have. I believe you have to dodge forward and have the projectile hit the shield at just the right moment for you to turn it around. Learn to dodge jump off of walls and such as well. Wall dodge jumping is extremely fast and very effective for getting out of the way of an attack.
I hope this helps a bit for those looking at getting into UT2004. If anyone would like to play sometime, I would be up for a game.