We’ll start out with the DO NOT’s:

DON’T aim at the floor/legs.
DON’T peek planted bomb instantly when you hear a defuse sound, wait 3-4 seconds and then peek.
DON’T hold down mouse 1 after you haven’t killed the enemy in 10 bullets, just strafe away and wait for your recoil to settle.
DON’T hold shift when the enemy already knows you are there or there is only one entry (B on inferno for example).
DON’T reload when your in a dangerous spot and you still have more than 5-6 bullets, get safe and then reload.
DON’T peek when your reload isn’t finished.
DON’T buy a helmet when you know the enemies either have AWP’s or AK’s and your money isn’t amazing.
DON’T eco when you have ~3500 on T or ~4000 on CT (again, don’t buy helmet) and you know the enemies economy is weak.
DON’T (VERY IMPORTANT) eco after you lose the pistol on CT or T (except for when you get a plant in the pistol on the T side). Buy helmet + pistol, scout or mag-7. You will be able to buy on the 4th round no matter what you do on the second round, and winning 2nd isn’t that hard considering their economy isn’t great either.
DON’T stand next to someone (especially awpers) who are peeking on a close angle. They want to strafe back if they miss their shot and if they get blocked, this usually means instant death.
DON’T rush in or peek with the bomb, just drop it in a safe corner where it can’t be seen by CT’s.
DON’T peek in with your W/forward key, always peek while side strafing.
DON’T run with a nade/smoke/flash in your hand, knife is 250 units/s nades are 245 units/s.
Now for some DO’s:

DO prefire common spots/corners at headshot level.
DO fake plant to get a chance at an easy kill, just hold E till you hear the sound and then spam 1 to get your gun out.
DO use shift walk when fake defusing (most usefull in 1 vs X), tap the bomb (tapping E to make noise) and shift walk into a wider angle so that when the T peeks the bomb you will be in a weird angle for him.
DO use counter flashes when you are getting pushed as a CT. When you are getting rushed, a smoke will not detonate in time to stop the rush. A flash however, will buy some time and maybe even get you or your teammate a free kill.
DO use molotovs to clear out common spots and then watch as they run for their lives while you kill them.
DO use incendiary grenades to buy some extra time (7 sec) for your teammates to rotate.
DO plant the bomb in the most optimal plant available. Most of you always plant in the safest way, although a better plant can give you the upper hand in a clutch situation. Learn the different plants and when to use which.
DO flash in for your teammates. Most of those flashed are easy to throw and don’t need to be learned. Just tell your teammate your flashing for him and he can get a safe peek and maybe even a kill.
DO learn the various T side smokes on the maps you are playing. YoutTube is a great source.
DO accept the fact that your teammate might be better off with the AWP than you are.
DO buy on the 3rd round if you got a plant in the 1st or 2nd round but still lost. You will have around 4500, should be enough.
DO learn the recoil patterns of the most common weapons (AK, M4A1), again YouTube is a great source.