Their are a few aspects to a team that makes the team strong on the Counter-Terrorist side of a match or scrim. One of these keys is good, efficient communication. One important part of this is making efficient and specific calls over ventrilo. You want to try to call the number of enemies guns used by the enemies, and specific locations of the enemies with as little words as possible. One huge mistake I have seen is a team member calling "ALL B! ALL B!" when only 4 where in B. This can be a big mistake because one of your teammates may try to flank and not be looking out for a Terrorists who was waiting in a great spot for your teammate to flank. Also your team should decide on specific names to call certain places and hiding spots around the map. This will help the whole team to know exactly where the enemies are when one of your teammates makes a call. Another thing that can be helpful is to call out flashes, smokes and the HE nades thrown by the enemy. This is another way to let your teammates know what is going on in your side of the map. One more thing is to cut the unnecessary talking on the mic. Try to refrain from talking about non-cs things in a serious scrim/match, and try to refrain from letting your whole team know how pissed you are every time you die. Lastly, let your teammates know when you are falling back, when you need backup, and when certain choke points have been compromised.
Solid Teamwork and rotations: The other major key to having a good CT side is great teamwork and good rotations. This teamwork is based a lot on good communication, however. Good calls will allow your teammates to make good decisions on when to rotate and how fast. You want to rotate fast when you know the enemy is coming to a certain bombsite or choke point. You want to try to meet them before they can even get into the site and plant. This is not always possible though. In cases where you can't help your teammates in time, you want to wait for the rest of your teammates to get into position to re-take the bombsite instead of trying to take the bombsite back one by one. Your in game leader can help out with this by trying to get everyone coordinated and grouped up. After you are all ready to go, you should flash the bombsite and all try to retake the bombsite at the same time.
Using HEs, flashes, and smoke grenades: All of these nades can have big advantages if used correctly. Using a flash is pretty easy, you want to use it to prevent rushes and to delay the other team while you wait for help from your teammates to help. Be careful, however, not to use your flashes wastefully. Good teams will try to delay their strats and get you to use up your flashes. This will leave them with a bunch of flashes and you with none. The HE, as a CT is often misused. A lot of players try to use it to get kills. You should use the HE to suppress the Terrorists. This will give you another nade to hold of the Terrorists when they make their move and also buy you a few seconds for your teammates to rotate and help. An exception to this is that I have seen many top teams use the HE to secure a vital choke point. An example of this is a two player nade stack on the long A doors on dust2 right at the start of the round. Lastly, the most underappreciated nade on the CT side is the smoke grenade. The smoke grenade can be very usefull when you are trying to guard a small choke point for several reasons. The first reason is because the smoke may force the Terrorists to waste flashes because they are afraid of what is on the other side of the smoke. The second, and most important reason to use a smoke is because you can see the Terrorist in the smoke before they can see you. A good example of this is the back alley or Ivy part of de_train. If you use a smoke in the little alley, you will see a part of them when they are still lost in smoke.
Know when to push: For the most part, you are going to want to stay back and guard the bombsites and choke points as a Counter-Terrorist. However, their are good times to push as well. A good flank can win you a round. Knowing when to push relies on two things: 1) How the other team is playing, and 2) communication. A few good times to push would be when the Terrorists are consistenly avoiding your choke point/bombsite or when you know where most of the enemies are. For example, if 4 of the enemies are rushing the other bombsite, it may be a good idea to try to push behind them and catch them off guard. When pushing, be very careful to check all the corners and good camping spots. You will most likely want to use a flash to help clear the choke point you are pushing threw as well.
Setup Crossfires when possible: This something that can be really helpful if you are able to do it. It is only possible to use on certain maps and certain choke points. However, when used, it forces the Terrorists to check two complete different directions in order to clear a choke point. One good example of this is having two CTs going to long A with one in the pit and one between the pit and the car.
Knowing when to Save guns and getting exit kills: Your team is not going to win every round, but their are a few things you can do that will help you win the following round. One of these things is knowing when to save your guns and when the round is unwinnable. Their are certain situations like a 2v5 with the bomb planted in which you have very little chance of winning. In this situation, it is statistically better to save the guns then to try to take the site back and lose both of your guns. By saving, you will have 2 guns and will be much more likely not to have to save. This is something that needs to be judged by your in game leader on a situational basis, but it is important that everyone is on the same page. You don't want 1 guy trying to take back a site and 1 guy saving. Also, when saving, it is a good idea to try to stay at a really save distance and to try to get some easy shots on Terrorists leaving the bombsite. Be careful not to lose your guns when doing this though. I will use another example from Dust2. A good idea, if you had an AWP and the bomb was planted at B, would be to sit in T spawn and wait for the Terrorist to try to exit from B tunnels. You will have an easy shot from their and also have an easy way to retreat if the Terrorists get to close.