These tips are meant for newer players. They consist of a small portion of my CS strategy.
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Learn the buy menu so you can go through it very fast. Also, after a while you'll know exactly what you can afford with the money you have. Don't always try to buy the expensive guns as you'll soon run out. If your team seems to be losing fairly often you're not going to make as much money. So, even if you do have a fair amount of cash play it safe and buy a cheaper gun.
You don't need to reload after every encounter, especially if you are traveling with a friend. Make sure the coast is clear then reload. There's no telling when someone will pop out unexpectedly while you're reloading and kill either you or your teammate. If you're getting low on bullets, pull out your hand gun. If you run out of bullets then, depending on the situation, you can either try to knife or reload if you think it's safe.
Travel with a buddy. Pick a friend and follow him around. Make sure he knows what he's doing, though. If you can trust that he's good enough, look opposite directions from where he's looking. If you're traveling down a long walkway, run looking backwards while he's charging forwards. If entering an area that could be potentially thick with campers, charge in with him and split up the duty looking in the hiding places. This normally happens at a very quick pace. Also, if he charges into a room and doesn't look around make sure you do it for him. Some players are smart enough to let the first few enemies pass by without shooting. It's your job to ferret them out before they hurt anyone. Lastly, if he is walking silently then do the same.
If your teammate is in front of you with a grenade out, make sure you keep out your gun and have a clear shot in front of him. He is vulnerable.
Never run/stand directly behind your teammate. Depending on how your teammate moves, weave back and forth behind him.
Important radio messages: "Fall back", "Need Backup", "Enemy Spotted" and "You take the point" (for when you're traveling with someone and you have low health. Have them go first).
If caught in a close quarters cross fire, strafe back and forth.
Shoot in bursts when firing across the map. Full auto is okay in close quarter situations, especially when you strafe. It's simply harder to aim and burst while you're strafing like crazy.
If confronting someone from across the map, don't end up playing a game of hide-and-shoot. Definitely do not wait in the open for the player to show themselves from across the way. If you do, do not stand/crouch in the same spot where he last saw you. Typically, it's wiser to fall back and go a different route. You'll likely meet up with them later or better yet, maybe someone else will take care of them.
Use your ears. If you're hiding and you hear someone coming (and you know they're alone) sometimes it's good to jump out shooting.
Make sure your speakers are set up properly so that you don't have them reversed and are hearing everything backwards.
At the beginning of the round use flash bangs in common travel routes. Flash bangs can be faulty, however, so never get so bold as to immediately charge in after a toss if you know there are more than a couple enemies there.
Use your radar. Assist people who intelligently use radio messages by traveling to their location. People flash on the radar when they use radio messages. Also, keep tabs on where your teammates are. When things are winding down it's a great way to locate the enemy if you suddenly see a friendly dot missing.
If you know someone's camping in a particular spot, charge around the corner with guns firing.
Experiment on your own with the recoils on the guns. Load a map up all by your lonesome and just shoot the wall with the guns at various distances. Note the bullet pattern for each weapon.Keep your cursor up at all times. Maybe even try keeping it at head level. If you are a less experienced player, try aiming at the chest. The recoil will naturally move your aim upwards.
Aim ahead of corners. If someone charges around a corner you're less likely to get a hit on them if you were aiming directly at the corner of the wall/box/whatever. Aim in front of it so that the enemy doesn't run past your cursor. Every extra split second counts.
If you're alone, stop every so often to check your radar and to listen. Walk silently often, if you have time.
Pick a gun appropriate to your situation. Shotguns and sub-machine guns for close quarters, rifles for expansive open maps.
Run with your pistol out at the beginning of the round. You move faster with your pistol. Also, if the bomb has just been planted, pull out your pistol to run there if it's far away and you feel safe.
In most situations, always buy armor first, even if you won't be able to afford a gun. If you play it smart you'll pick one up along the way. If it's the beginning of the map and you're a terrorist, however, I'd suggest buying a better pistol. The glock is horrible.
Play in according to your map objective. It's not okay to camp when there are hostages to rescue, but it is okay to camp when there are hostages to guard. It's not okay to camp when you have to plant the bomb.
If the bomb is down and you're a ct, guard it. If the bomb is down and you're a terrorist, gather as many friends as possible and then check every camp spot. If you're all by yourself walk silently and check the camp spots.
If there are enemies using the AWP, buy a smoke grenade and use it strategically to get to where you want to go.
If you've got time and you go to defuse the bomb, click to defuse it and immediately bring your gun back up. Terrorists will hide and wait to hear that sound before they pop out and shoot you. If you're a terrorist, be aware that the ct's will do this.
If you're caught in fire by surprise then jump! It will confuse the newer players and it gives you time to spin around in the air looking for your attackers. This of course requires good mouse skill and technique. Sometimes it will throw off an experienced player's aim too, even when you fall back down and crouch.
If you're engaged in fire while standing up, crouch down. Hopefully, that quick action will throw their aim high for a split second.
Sometimes when someone catches me unaware in close quarters, I get surprised and my cursor ends up too low on my target (legs or groin). If this happens, shoot the enemy in an ascending line. Hopefully you'll be quick enough to get a headshot off on him.
Lastly, avoid being predictable. Do the unexpected. That's why llamas get kills on really great players. They do things that make no sense whatsoever.