When playing a dukematch there are simple things you need to remember to stay alive:

Never stop running!!! - A moving target is harder to hit. I must emphasize this point because its the hardest thing for new players to pick up. New players tend to stop, turn, and shoot. You should never stop at all. Try to be as fast and unpredictable as you can. Also, be sure to run and not just walk. You can turn the auto-run feature on (default is TAB), or simply hold down the run button the whole time you are playing. 

Learn to use the strafe key!! - The next thing new players are taught to do is strafe (sliding from side to side). I have seen many fellow duke players get a lot more kills and dodge a lot more bullets simply by strafeing (default is ALT). By strafeing, projectile weapons like the Rocket Launcher and Shrinker will usually miss you. Also, when strafeing, constantly fire your weapon for a higher hit ratio. These first two tips are the keys that will keep you alive.

Jump and Duck! - Again, I must emphasize the importance of moving. Another dodging tip is to jump and duck while running. Jumping has kept me alive through many rocket firing enemies. When you jump, any projectile will usually go beneath you and your enemy will have a harder time keeping his site on you.

Watch your health and armor! - If you don't have a lot of armor and your health is low, you will eventually die. I never attack unless I have at least %100 of armor and health. Of course, on some levels health and armor will be scarce, but use common sense. Don't attack with %50 health and no armor unless you know he has less. With full health and armor, you can sustain a heavy amount of damage, including a direct Rocket Launcher hit.

Don't use the Devastator - Contrary to it's name, the devastator is anything but devastating. It does, however, make a lot of scary noise. It fires too slow and uses too much ammo to be a practical weapon. You will be better off going for a Shotgun, Shrinker, or Rocket Launcher. Only one situation (other than suicidal ones) will make use of the Devastator. Read on to find out when to use the devastator. 
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Learn to use the Jetpack - The ultimate defensive item is the Jetpack! Any projectile is almost useless when you are flying. Only two weapons can successfully take down a flying enemy, the Shotgun and the Chaingun. You quickly find that the Jetpack and Shotgun can be a deadly offensive combination as well.

Learn to use the Shotgun - You will soon find that the most experienced players will use the Shotgun. It is the strongest weapon with an instantaneous hit. From a distance the Shotgun's power is lost, but at close range it is deadly. Many top notch dukematches result in both fighters jumping around firing the Shotgun at each other. The Shotgun also has the widest firing area. It seems you can sometimes hit your opponent even when your not exactly pointing the Shotgun at him!

Turn and Shoot - Your enemy shoots your health down, you armor is all but gone, and you are screaming in pain. Most people run for dear life, and that is good to a certain extent. Chances are that when you run, your enemy will follow and try to kill you. However, I have gotten many kills by going around a corner, turning around very fast, and firing a couple rockets. One good example is on the very first level, "Hollywood Holocaust". There are many turns and hallways inside that will allow you to hit an unsuspecting opponent coming around the corner. 

You Shrink em', you get em'! - No question, my favorite weapon is the Shrink Ray. Its the only gun where one hit leads to a kill. When you hit your enemy and he begins to shrink, quickly run toward him. He may try to counter with the Steroids or a Jetpack so be alert. If he uses the steroids before you get to him, your pretty much out of luck. Try using the chaingun to wear him down while he is running. If your shrunken enemy starts to use the jetpack, quickly switch to the....well....the Devastator. This is the ONLY instance in which the Devastator is of use. Because your opponent is small, he moves and flies at a slow pace. Just fire the Devastator at him in the air. He will not be able to dodge the missles that will be flying all around.

Don't get squished! - When you are shrunk by an enemy, don't fear! There are several ways to get away safely. The most obvious is to use the Steroids. If you have them, use them fast!! If you do not have the Steriods, hit the Jetpack. It is much harder to kill you in the air than on the ground. If your opponent tries to use the previously mentioned tactic, quickly turn off the jetpack and fall to the ground. You should be able to buy yourself some growing time. If all else fails, try to run in circles or head for some water to hide in.

Brighten your screen! - You can't hit what you can't see, so why not see a little clearer? When your enemy hides in a dark area, you cannot see him; however, if you increase the screen brightness to 9, you can see his outline!! It may not look pretty, but it could save your life and add a kill.

Get the highest frame rate! - You will be able to aim better if you have a higher frame rate. I have found that the VESA 2.0 compliant 320x200 mode offers 10-15 more frames per second then the "Normal Mode" 320x200. Your video card may be VESA 2.0 compatible already or you may need an external driver like univbe.