Study the maps. You'll get nowhere if you just run around without knowing where you can find good hiding places, sniper spots etc. The best thing to do is to create a new server and explore the map on your own.
Advanced Dod players can kill you before you can say "Sh*t!". Therefore, learn to look in such way that you can oversee coming situations. For example, if you go through a door, it's best to look in the direction where you're going and strafe until you can go this way. Also, put your mouse resolution as high as you can, so that you can react quickly.*
Reload whenever you get the chance. If you get caught with an empty clip, you're probably going to die. Result to bunny hopping and avoid melee.
Remember that the assault class has a sidearm in addition to a submachine gun. If you're fighting and your clip runs out, pull out the pistol. It won't stop a whole wave of enemies, but it might let you survive long enough to get away and reload.
Specialize in different classes. If you only play as a sniper, you'll have a very hard time in maps like dod_flash. A good exercising map is dod_avalanche, because there's a balance between close and long-range combat. You need to be able to use all weapons and any given time for instances like scrims and realisms.
Learn to left peek: Works great for all classes from heavies to rifles to snipers. To left peek is easy, go to a doorway. Next, move yourself to the right side of the doorway so your crosshair is level with the edge of the doorframe.
Never be afraid, or ashamed, to camp for long periods of time. Eventually someone will run into your view and you will score 1 kill iyou need to be patient during scrims you all get anzy and leave your positions taking down our whole defense without even realizing it.
If you see one. . .
There’s probably more right behind. Don’t drop your guard after you turn a corner and kill the enemy. More often then not, there’s another close by
While this is an vital part of an FPS, the truth that this comes through practice. There really isn’t anything I can offer here except the confirmation that this will only come through more effort. That being said, if your mouse sensitivity is around the low single digits, you should rack it up to at least around seven. The reason being is that oftentimes you will have only have a second or less to react and pull off a shot. Thus it is imperative that you get your crosshairs over your target ASAP
Another attribute that comes mainly from effort and time. Some advice I can offer is playing with more focus, or becoming so absorbed in the game that you forget what’s going on around you.
The only class that ever has a real excuse to be going prone frequently is the Machine Gun. Riflemen, Support, Assault, should be relying on crouch if they need cover unless there is no way to proceed safely unless they go prone. The reason is that prone severely kills your mobility while offering your class little in the way of benefit. Even Snipers should stay out of prone as much as possible. If you are relying on the prone to help your aim, you not only hurt your options when choosing sniper points, but it will hurt your ability to stay alive when that rocket or grenade comes flying in, or a full fledged rush comes through.
Don’t grab the ass of the person in front of you. The most obvious reason is that if a grenade lands near the person in front, its going to be two kills instead of one. Another situation is if someone pops around the corner and hoses down the hallway. You don’t have a shot, and all the enemy has to do is point and hold down the trigger for another free kill. As an added bonus, if you do keep your distance from your front man, you get to benefit from his death as a free MG/Sniper bait.
For sniper/MG
1. The best spots allow you to cover a single choke point without having to worry about anything besides someone coming to ream you from behind. If you have to worry about two spots, that’s not so good. If those two spots require you to shift your crosshairs, that’s bad. As in, get the hell out of that spot as soon as you’ve done what you need to.
2. Shift your position. After several shots, pick another spot. People eventually learn about sticking their head into dark holes and getting their heads whacked. Sooner or later, they’ll start pouring napalm down the holes first and then poking in. Keep moving to keep them guessing. Unless it’s a real primo shooting fish in the barrel type spot.
3. Keep an escape route handy. Especially because trouble usually brings backup.