The first in my new pro tip series:
Always use a dot on your screen. In so many games it is vitally important to always know where the dead centre of your screen is. So many games have weapons (ie sniping, throwing nades, SRAW) that have the crosshair removed.
BF2 for example, you could fire and guide the SRAW without zooming in, and the missile would track the dead centre of the screen and because you wern't zoomed in, you were much more mobile. Also for BF2, when sprinting there is no crosshair, but when you use a dot, you can start lining up the shot perfectly before the crosshair appears, saving you a lot of time and also allowing you to jump in the air and 1 bullet HS people the instant you land (because you lined the shot up mid flight, instead of having to wait for the crosshair to appear).
Using a dot is also very beneficial for strafe scouting in CS.
I find the best way to put a dot on is to load up CSS, create a custom game and buy a scout. Affix a small blob about 1mm wide to the centre of the screen (using the scope as a guide) using the tip of your fingernail. Also, every shooting game I have played uses the exact same dead centre of the screen (thankfully, otherwise moving your dot around for different games would be aids)