To improve your performance, pay attention to details, and importantly to the signals your body gives you. Make sure your body and mind are in top condition. Being tired (or too full of energy), being too hungry/thirsty, feeling too much pain in your thumbs from overplaying, or having cold hands (or being too hot and sweating too much) can all be reasons why your performance is decreasing. Find the factors that are important to you, and make sure to be at 100% for your play session.
Some people can play CTR fine without sound, and even listen to music or other distractions while playing. In general though I recommend playing with sound. Sound clues give you more certainty with what is happening with your driving. For example, listening the powerslide sound, you can hear when you miss it. You can also hear if you land with a hang-time turbo or not, and you can hear when your reserves go out. You can also time your full turbos simply by hearing, instead of looking at the turbometer filling up, which is important because you should be looking ahead at the kart and/or the track in front of you, not too much at the bottom right corner. A good idea is to lower the Music volume to 0% in the settings, in order to only hear kart engine sounds. The music might disturb your powerslide rythm. Playing with headphones to completely block out distracting sounds can make a difference, especially if you live in a noisy household.
Play in a comfortable position to avoid back pain. A good chair and the TV at the correct height are key. It is recommended to make frequent breaks (maybe 10-15 minutes every 30-40 minutes) to keep mentally fresh, and to refuel/get your blood flowing. Don’t press buttons too hard as it can give you pain more rapidly and has no use, unless you’re playing with an analog PS2 controller. Try to remain motionless, to focus your mind only on your fingers. I’ve seen new players turn their body in the direction that they want their kart to turn, and it’s a bad habit. Frequent physical activity can be important. If you’ve done two great laps and start feeling stress in the third, having extra stamina can make the difference. In that situation, remember to breathe! Panic, and the mistakes that come with it, will arrive much sooner in an oxygen-deprived brain.
If you have a memory card, you can race against ghosts. Ghosts give you an indication of how well you perform during the lap, and also at the end of the laps, so you don’t have to look at the timer. Besides, ghosts also help you find the right trajectory, and see the mistakes you’ve made in previous courses if you didn’t film it. However, it can be stressful, especially if you’re not used to it. Ghosts can even be used for lap times, although it can be tricky to try to start the lap at the same time as the ghost.
Don’t spend too long on one track, change tracks often. If you find yourself unable to improve, it would be a good time to switch tracks instead of trying desperately to reach a certain goal. Keep in mind that your skills will improve over time, and a fixed goal (for example, sub 1’13 in Roo’s Tubes) that requires a lot of luck for you now might be much easier and require less luck later. Always aim for times that don’t require a big amount of luck (and time) to do. Come back and improve your time when you’ve improved as a player. Many advanced techs are very hard to do, but don’t get discouraged too easily! If you couldn’t do a tech the last time you played a track, try it again next time you play it. Always prefer improving your times with improved techniques, as opposed to improving due to having had more luck in your run. Don’t focus too much on reaching perfection, as even if you reach it, new techniques will still probably make it obsolete. V4E’s 1’52”71 in Oxide Station is an almost perfect time, but it is not WR because a new SC made it obsolete. You will generally climb faster up the rankings by improving your weak tracks than by improving your best ones (if your goal is to be ranked as high as possible, that is).
Don’t forget to restart frequently. Don’t continue runs that aren’t worth the effort. Even many pros often do this mistake. The time saved by restarting immediately instead of continuing for another 10 seconds is invaluable. Try to acquire saffi fire as fast as possible and, in fact, limit yourself to the amount of reserves that you actually need. For example, you don’t need saffi fire in Turbo Track lap, some good reserves are enough. The goal is to have as many lap attempts per minute as possible.
Don’t forget to spread your time between playing CTR and watching videos of CTR, especially videos of the best players. The learning process is a back and forth between both. Don’t hesitate to ask other players for advice, and to publish videos of your driving, even if you think your videos are bad! The comments that you may receive can help put you in the right direction, as others can see things that you didn’t see or pay attention to.