Aiming is grouped into two different kinds of movements: tracking, and flicking. Being proficient at both is important, although certain classes and characters in fps games will be more dependent on one over the other.
Tracking is the most basic, easily understood type of aiming. Tracking is essentially what it sounds like: following a target with your cursor as it moves, reacting as quickly as possible to changes in direction to keep as close as possible. Tracking becomes more difficult in fps games the closer one gets to a target. Fps players who enjoy using close-range automatic weapons (like, for example, Tracer in Overwatch) lean heavily on tracking skills to keep applying damage to a fast-moving target.
Flicking or flick-aiming is a bit more complicated, but also incredibly useful. Flicking is used primarily by semi-automatic weapon using classes like snipers to make very fast, linear mouse movements to “flick” to a target’s head. Flicking requires very precise mouse movements because the targets of flick (usually the head) are often very small, and requires the ability to predict the trajectory of a target. Flick shots are performed as fast as possible to minimize the distance that a target moves from when it is first seen to when your mouse is on top of it. Flick aiming requires consistent, long –term practice at a set dpi to engrain these high-speed movements into muscle memory.
Combining these two movements seamlessly is what makes pro fps players so skilled. Flicking benefits from tracking by allowing the player to follow their target before flicking, minimizing the distance that they must move their mouse during the flick to reach the enemy’s head. Tracking gains from flicking by giving the player the ability to quickly catch back up with a target after it has made a sudden direction change. Both require snappy reaction times and, most importantly, lots of focus if one wants to both aim effectively and improve.
While the largest improvements in aiming one will experience are going to come from pure practice and patience, there are some things that can be done to speed up the skill building process and avoid handy-capping yourself.
First, give yourself plenty of room to move your mouse. Most pros have large or even very large mousepads, allowing them to keep their dpi low without constricting their movements. Try and make room on your desk for you to move your arm and shoulder more freely, and consider buying a larger mousepad (they’re quite cheap; mine was $10 on Amazon). Making room may also include moving closer to or farther away from your monitor so that you aren’t straining your eyes to see the entire screen.
Second, get the best monitor and rig you can afford to maximize fps. While you can improve at aiming playing at lower framerates, this means that the target you are trying to aim at is literally on your screen for less frames than someone running the game at a higher framerate. This results in choppy visuals, making tracking in particular more difficult. If you can lower game settings to achieve, higher framerates, I highly advise doing so. Many pros play their respective fps game on the lowest quality settings to both boost framerate and cut down on visual clutter like explosions effects and foliage that simply distract from aiming. I also made a post on high-refresh-rate monitors and their benefits, which you can read about here.
Lastly, and most importantly, FOCUS. Improving your skills in aiming demands your constant attention. If you aren’t keeping yourself attentive, thinking about the shot you are about to make, you risk dropping into auto-pilot mode. This means that you may still be aiming, but your brain isn’t really paying attention to the movement of your targets, what you did wrong, and what you did right. Going into auto-pilot is really a waste of time. If you’re too tired, either stop playing and get some rest or choose a character or class that doesn’t lean on aiming ability. In my own experience, a single hour of highly-attentive, focused gameplay is worth an entire day of unfocused, lazy playing.