1. Cover and Reload
A. Cover when reloading
Your most vulnerable when your reloading, it is most important that you get behind a wall or box whilst you are reloading, not only will the enemy not see you, they won't kill you. Many players do not run for cover whilst they are reloading and stand around waiting to be shot at with no hope of shooting back in time, don't let this happen to you.
B. Don't habitually reload after a kill
Many players try and reload their guns straight after they kill an opponent, this is a fatal mistake that gets many players (even the pros) killed. The reason you do not reload straight after you kill an opponent is because there is a possibility that another opponent has come in straight after and if you are reloading, you will almost certainly die.
2. Smoke and Flash
A. Use smoke grenades to provide cover when none is available
If your reloading and there are no walls or boxes around you to give you cover then I suggest that you should use a smoke grenade and block the enemies sight of you. Using a smoke grenade is also a good idea in situations where you want to distract an opponent or draw them into the smoke. If your opponent is aiming at the smoke it will take them some time to find you in it before you kill them.
B. Use flash bang grenades before charging
Using flash bang grenades before you run in can be a life-saver, this is crucial if you know that several enemies lurk around the corner. I would always use a flash bang before charging into the enemy base.
This ends my tips on how to stay alive in Counter-Strike