Keep moving.
Always stay on the move. A stationary target is a dead target, and you never know when there's someone on the other side of the map using a sniper rifle to draw a bead on your melon. With very few exceptions (see the next tip below), don't ever stop moving, especially when in a firefight. Learn to use the mouse so you can aim while moving around to dodge your opponents' attacks.

Keep crosshairs at head level.
When walking or running around, keep your crosshairs at around head level. This way you'll have a better chance at a headshot if someone jumps out at you. It also decreases the time it'll take to draw a bead on someone's noggin.

Jump around corners.
When rounding corners, jump into the air and turn to face the new direction. This will make you harder to hit (especially with a headshot) to anyone lying in ambush on the other side.

Switch to the pistol when needed.
Don't underestimate your sidearm. If you run out of ammo or have to reload in the middle of a firefight, switch to your pistol and unload a few rounds while retreating. It may just save your life.

Cut your teeth right.
When starting out as a Counter-Strike newbie, get used to weapons like the MP5. It's an automatic weapon, fairly accurate, and will serve you well to build up your skills before tackling other weapons.

Master the crouch jump.
Pressing jump then crouch is an important technique to master. It'll allow you to jump onto taller obstacles, and will let you jump through narrow windows or gaps. Practice it and learn how to use it well.

Always be unpredictable.
Don't do the same thing over and over. Opponents learn fast, and good opponents learn really fast. If you score an easy kill or ambush one time, don't do it again right away. Take alternate routes through the level occasionally. Being predictable will quickly get you killed.

Stick together, team!
Always travel in groups. The odds of survival are greatly increased when you have backup. At the start of the level don't go rushing off by yourself. Stick with your teammates and stay alive.

Learn the level and objectives.
Know thy level. If you don't, glide around in spectator mode to learn the layout and objective locations, like bomb points and hostages. Pay attention to where other players go and pay attention to the layout of a map. Knowing the quickest way from point to point is key for success.

Rush the sniper.
When you spot a sniper, don't try to outshoot him. Either avoid him if possible, or rush right at him while moving erratically. Most snipers will freak once you get up close, and you can mow them down.

Use grenades.
If you can afford them, always buy a grenade at the start of a round. Toss them around corners where you suspect opponents lie, and use them to stop rushes and weaken opponents. Be careful with Flashbangs, though, as they can blind you and your teammates as easily as the opposition.

Balance your team's arsenal.
A team needs a mix of weapons. So, if you notice everybody on your team equipping to play the sniper role, buy an automatic weapon to help them out and provide cover.

Buy armor.
Don't forget to buy armor and a helmet. It'll greatly increase your chances of survival.

Always go for the headshot.
Headshots do lots of damage. 'Nuff said.

Fire in bursts.
Especially for weapons like the AK-47 that quickly lose their accuracy in automatic mode, firing in quick bursts is essential. Just tap the fire button/key quickly, then tap it again.

Cover the bomb carrier and VIP.
Always provide protection to the bomb carrier or VIP. These people are essential to your survival, so keeping them alive pays off. If you are the bomb carrier or VIP, then don't go rushing off on your own.

Check for ambushes before disarming bombs.
When you spot a planted bomb, don't rush right in and try to disarm it. Clever Terrorists will lie in ambush and try to nail anyone made vulnerable by bomb defusion. Always look for traps first.

Use cover and dark to your advantage.
Hide in dark corners and behind obstacles to make yourself harder to hit and see. This is especially important when sniping.

Listen up!
Use sound to your advantage by listening to movement sounds. This will often allow you to pinpoint the location of opponents and avoid running into them headfirst. You should also learn to crouch or hold down the walk key (shift by default) to mute your own movement. Finally, use headphones or loud speakers to make things easier to hear.

Shoot through walls.
Some weapons can shoot through some walls. Learn which weapons and which walls so that you can use this fact to your advantage. If you see or hear someone duck behind a fragile barrier, you can put them down with total impunity.

Use hostages for cover.
Opponents who kill hostages earn a cash penalty, so using hostages for cover is a good, if cruel, tactic. Of course, it makes it hard to shoot back yourself...

Reload before running out.
If you can help it, don't wait until you're totally out of ammo to reload. And if you can, seek cover while reloading, since it makes you quite vulnerable.

Buy a better pistol.
You may often have to rely on your sidearm, so upgrade it if you can afford to. It makes a lot of difference.

Learn radio commands.
Use the radio commands quickly to communicate efficiently. And don't hesitate to respond to them if you hear them. When someone says "I need cover", they probably mean it!

Switch to lighter weapons to run.
Readying lighter weapons like pistols and the knife allows you to run much more quickly, so it's a good strategy when you need to get into position quickly --like at the start of a level.

Snap out of sniping mode
When you're zoomed in on someone, quickly double tap the "next weapon" key to snap out of zoom mode, switch to another weapon, then switch back to the sniper rifle. This is great if you need to move or dodge quickly.

Watch the Crouching
Be careful when you duck down for that acuracy shot because those who don't duck usually keep their crosshair at waist level. Most people also aim for the biggest part on their opponents body. It always being the shoulders to the waist. Or if you are battling it out with another player standing up, don't duck - you're sure to get shot in the head.

Duck and Jump and Hop
When jumping to make yourself a harder target, crouch as well. This makes you an even smaller target

Gimme a Boost!
You can get to high places by "boosting". One player should crouch and the other one should jump on top of him. If you need to get even higher, the player above can jump, and the player below should let go of his crouch key.

Gamma Correction
Turn your gamma up, which allows you to usually see players hiding in darker areas. (ie, dust, in the hall right next to the 2 stack of boxes...)

Flashbangs are Better
ALWAYS buy flashbangs.. In my experience/opinion they are far more effective than an H.E. or smoke.. Throw them into a room where badguys will most likely be, or save them on you and use them if/when you get pinned down, timing is critical so that u don't flash yourself! Rush in RIGHT after/when it goes off and shoot the blind bastards!

Circle Strafe
Use Circle Strafing when in a close combat situation... Circle strafing (for those who dont know, is when you strafe around your opponent always facing them).

The Old Bait 'n Snipe
If you're gonna snipe or hide, always do it in the least expected place. Hide in vents, or over doorways, and have a free shot at anyone that passes underneath. You could also "bait" someone by leaving a hostage out in the open and hiding nearby in the shadows. They'll rush for the hostage, right into your line of fire.

Don't Give it Away
Use team chat! It can be useful to organize your team or to decide what to do next round. Far too many people blurt out their tactics over public chat, leaving the enemy with a good opportunity to counter it.

Give 'em a Hand!
Don't block your teammates' line of fire, don't block their line of movement while retreating or strafing, and if a teammate is being shot at, don't just let him die. Toss a grenade if possible, and join the fight. Because if they kill him, then they're coming after you next. Better to stop them sooner than later.

Bind New Commands
Using the console, I would recommend binding keys to quickly say messages that aren't included in the list of radio commands. For example, you can key-bind q to say "STEP BACK, GRENADE COMING UP!" or "Walk, don't run". But make sure it's configured to "say_team", not just "say"

Go For the Crotch!
When firing on full automatic, aim for the legs or the groin or something. This is because the recoil will kick the bullets higher and higher, so your 2nd or 3rd or so shot will land on the neck or the head. If you're close and sure of your accuracy, then by all means, aim straight for the head. But for long-range engagements with a weapon that has no scope, aiming lower will help.

Hostage Jump
On some cs maps, if you're playing with a buddy that you can talk to (like at a LAN party), you can pull off a "hostage jump" that'll get you into hard-to-reach places. Have the other guy crouch while he's next to a hostage. Then, jump onto the guy, then jump onto the hostage. Now, have the other guy lead you wherever you want to go. For example, using this trick, you can jump onto the light fixtures in the ceiling of assault. Here, you're pretty much invisible and able to fire with impunity onto any CT that comes in.

Hide Hostages
In a Hostage Rescue Map, as a Terrorist, take a hostages with you to the most isolated and hidden spot you can find. That way the CT's will have a harder time finding the hostages and it will be easier for you to win (just remember to guard the hostage).

Use smoke Grenade AND Flashbang
Try to use a smoke/flashbang combo. A lot of players know to duck back if they see a grenade coming to avoid the flashbang. Throw the smoke grenda first, once the smoke screen has deployed, toss the flashbang. Your enemies will never know what hit them.

Do Not Type
Never type unless you are dead. It is a horrible habit to do so! If possible, use Radio Commands instead, because if you type, it makes you very vulnerable, because first, you are not playing, and second, it gives your opponent 2 seconds to kill you.

Try to Snipe "fast"
The key to a good snipe isn't in a slow aim, but to shoot quickly before the target has a chance to react. Many times, a constantly moving target will make you wait until your bead is sure. By the time you're able to take a good aim, they're usually in firing range of you, and you've lost your advantage. Also remember that the AWP is usually one shot / one kill, and that the Scout is usually two shots / one kill. Hit them fast and you can't go wrong.