Aiming improves your accuracy a bit but it slows you down, as you may know. Crouching is only good if you like to go spelunking in the vents.
Practice makes perfect, as with everything. Try to aim your shots by adjusting them based on the bullet tracers. getting used to knowing where the centre of the screen is helps as well but that takes time and a bit of help from what I just mentioned. You can also use the radar or the timer to adjust your aim on the horizontal plane. Keep in mind that silenced weapons don't have bullet tracers so those can be a bit harder to aim with at first but, as I said before, once you get used to knowing where the centre of your screen is, everything will be easier and second-nature to you.
I usually only use the aim mode when the target's really far away.
