Here are my pointers. I'm an average aimer, but used to be a total newbie (read: could not hit anything). But, nowadays I can do most things, with the exception of middair mortars/electros (I'm also useless with the laser.

).
Anyway, I want to preface this by saying it's important to point out some people have aiming talent and others don't. Just like in any ability. Those with incredible aim have a talent that most of us don't have. That said, there's still a lot you can do to maximize the ability you do have.
1) Good aiming is instinctual, and based more on your hand/eye/mind reacting, as well as practice. If you relax and let the gameplay "sink in", you'll start to see how people tend to move, and how to aim and fire each weapon. Conversely, if you're thinking a lot about each shot, where your crosshair is, and theorizing the possibilities of where your opponent is...you're already dead.

The game moves much too fast for too much thinking!
2) Have fun! If you're stressing by not being able to hit anybody, or by dying so much, that's not good. Take a break. When you come back, your aim should be better just because you're more relaxed.
3) Get good at staying alive. You can't shoot if you're dead already. Know a map, how to get the weapons/armor/health. And keep moving!
4) This ties in with number 3. Don't "fall for the bait." Some guy attacks you, and you have a good weapon and high health, and you're determined to kill him. So you have a duel, forgetting the fact there's 10 other people on the server, some of which are likely to round a corner and kill from you behind. When you get good, you might enjoy "hunting people down"

, but as a newbie I think it's much more useful to practice your ability to be hard to hit and unpredictable. Take shots at people as they come, but as soon as things get hot, you might want to leave the firefight and get more health/armor. In other words, the longer you stay alive, the more weapons/ammo/health you'll have, and therefore more opportunities to shoot at stuff.
5) Don't track a target with your mouse. It's very tiring on your hand, and is a very *conscious* effort. If you watch good players/aimers from first person (go to spectate mode), you'll see their mouse often seems to "jerk" from one point to another. Counterstrike players called this the "flick"...in other words, you don't move your crosshair over your target, you just flick your wrist the approximate distance you think would put your aim dead on and
fire. You don't even let your eyes confirm whether or not your aim was correct. This saves time/energy (very important!), and makes your aiming much faster. After a while, your visual/motor system will start to automatically have a feel for how far the flick should be. This is how any type of pro gamer, or any athlete for that matter, reacts to very precise and fast stimuli: instinct.
6) Have a comfortable setting for your mouse sensitivity. Everyone's different (some like it extremely sensitive, others less so). Just make sure you can do a 180 degree turn around quickly without straining (i.e. having to pick up and move the mouse or, in the other extreme, "overturning" and making a complete revolution). This basic setup is what's allowing your opponents to turn around when they're being hit from behind and quickly return fire. Gamers make way too much a deal about mouse/mouse sensitivity though (go to any gamer forum and you'll see thread after thread on this topic). Don't obsess about your sensitivity. Your visual/motor system will get used to pretty much anything as long as it's comfortable.
7) Your mouse is only one half of your aiming! Use the WASD keys to strafe into shots, or (less commonly) jump into them. This is often a good technique when you have someone who is *just* outside your crosshair...sometimes it's easier to tap A or D instead of moving your mouse. Give it a try.
8 ) Use different weapons in a fight. This is another general gameplay tip, but does help your aim as well. A newbie always sticks with one weapon, but you will see better players switching weapons based on which would be more effective, and also to do "combos" (e.g. knock someone into the air with the Mortar, and then Nex them in middair). The more you are aware that your weapon is the best or worst for a given situation, the more you will change them up and thereby kill two birds with one stone: become a better player but also become comfortable with aiming with the different weapons right after one (like in my example above with the mortar and nex).
9)
fraggot wrote:there must be a trick other than using the crosshairs to aim. especially when dancing at close range with matched weapons and getting pwnd every time.
I feel you Fraggot! Been there, know how you feel. This probably has more to do with your opponent doing a better job of avoiding your shots, and you being unable to predict where his next move will be, rather than anything to do with aiming. Also, in "dancing" it's easy to get flustered and start spamming shots instead of paying attention to your opponents evasion techniques. Finally, in close quarters it's easy to get lucky: you'll see better players will avoid being really up close if you have a weapon such as the rocket launcher, because you could just fire at the floor and hit both of you. A little distance can help you draw a bead on the target. So, don't dance unless you know why you're doing it (i.e. you have the advantage, or don't have a choice etc.).