There are just a ton of very little things you can do to improve your play. They fall under different categories such as hardware setup, software setup, mental and body preparedness, in-game tactics, and learning.
So, for your hardware setup, you just make sure you have a decent system that can run the game smoothly with or without software optimizations. It's good to have a decent mouse and mousepad, preferably with a larger than average surface area. Other things that can help that you some might not be aware of include: a good chair (preferably without arm wrests that interfere with arm movement), proper posture and height to keyboard/mouse, proper positioning of peripherals, etc.
With your software setup, again you want to make sure things run smoothly. Turn off any unecessary background programs. (it helps to be the only person with programs using the net at the time) Setup your config to remove ragdolls and improve your rates. Etc. Try experimenting with lower mouse sensitivities, especially once you have a good mouse and mousepad. I have a 2.5 sensitivity. It increases your aiming accuracy for that first headshot, but if you get it too low (and especially with a smaller mousepad) it gets difficult to turn and hit someone on your flank.
With mental and body preparedness there's a few things that can hamper your ability to aim with precision. It's not good to drink caffeine or even excessive amounts of sugar before playing. Your hands should not be cold. Sometimes you'll be slow for one reason or another (some suspect that eating right before playing can do this). I find it helpful to forcefully change my mental state by using the Scout and flying around the map pretending I was the best player in the world or something. I shoot before I think; force myself to fire before I'm ready; push the limits of my speed. This wakes me up and focuses my attention.
In-game tactics can include an almost infinite amount of tips on it's own. As for generalizations, movement is often under-valued. Everyone says aim is really important, and it is, but movement is often what sets apart a good player from a great player. Knowing where to place yourself and when, where to look first when charging, knowing when to engage and when to back off, knowing when to rotate and help your teammates or stay in position to watch for a fake -- these are all things that can help improve not only your K: D ratio but your team win ratio, which I find more important as a determination of success. If personal experience just isn't helping you learn these tactical skills, the next category has you covered.
One of the best things about CS that people still complain about is spectating. People may hate waiting to get into the game themselves after they die, but there is value in watching other's play. I like to take this to the next step and watch competitive matches, even if they're for CS 1.6. You can even record a demo of yourself playing and watch it again. See what you messed up and make a mental note to change. It might not work at first, but you'll steadily improve because everytime you mess it up it'll be smashed into your head that much more. It helps to have a learning mentality too. Everytime you die, think "is there anyway I could have succeeded in that situation?" Think beyond "I could have hit him". For instance, on the offensive, you can win CS without having to kill anyone. How does this improve your K: D ratio? Well, it gets you better weapons and you don't have a death. It also builds confidence and helps you learn enemy manipulation techniques like stalling, faking, baiting, distraction, and in general picking off people one by one instead of coming face to face with 3 enemies. (shoot before you see them if they already know you're there)
I hope you derived some helpful tips from my blabbering, but this is the kind of stuff that I've learned over time by myself and from others. Here's a somewhat old but still quite valid and detailed guide to CS competitive play. (Don't bother reading past when it says "Very Basic Buying Strats". That's about half-way down. Disregard some of the specifics that don't pertain to you.)
http://whisper.ausgamers.com/warstrats.htm