There is nothing quite like a narrow runway to make you focus on placing your aircraft slap bang in the middle.
It helps to know what that looks like from your perspective. The home strip where I teach is 30 meters wide, so some student pilot encouragement to maintain the centerline is often on the cards. We practice this before every take-off and after every landing.
The problem is that most light aircraft have side-by-side seating, so placing the nosewheel, (or the tailwheel), on the centerline is not all that simple at first.
If you were sitting in the middle, its simple... the centerline must be right in the middle, between your legs. Everything lines up.
Here's the trick to figuring out the centerline when you are sitting side-by side:
Ask your Instructor to show you the center "picture". No Instructor? No problem.
Maneuver your aircraft with the nosewheel perfectly onto a line with your tow bar. You will usually find a centerline on the Apron or taxiway somewhere. Climb into your seat. Sit straight, and line up a rivet or some other indicator on your aircraft nose cowling with the centerline. This is what "in the middle " looks like from your perspective.
Strangely, the centerline always looks like it is more or less between your legs. I find that in the Cessna 172, from the Pilot seat, the centerline looks like it lines up with my right inner thigh. The same seat in the Sling 2 has the centerline running neatly along my bum crack.
Double check your center view, (to satisfy your natural sense of the impossibility of this picture), by moving your face slap bang to the center of the aircraft. You will see the center stripe in the middle of the aircraft. Move back and sit straight in your seat. Get this mental image set in your mind.
Yes, I know it feels uncomfortable and all wrong, but eventually, with practice, it will feel comfortable and correct.
Whenever you taxi, put your aircraft nosewheel on the centerline to practice, based on this "picture".
When you stop, if you can, shut down on the line, get out, and check your nose wheel position, or get a friend to video you taxiing in. You will see if you are spot on or a little off, and can fix it.
Approach on finals with this same picture, even if it feels weird to you. You will usually feel like you are too far right, (from the left/pilot seat). Eventually this "weird" feeling will transform to become your natural "centerline" picture. Most student pilots land left of the centerline.
To torture my students for wandering off the centerline, I sing them my "centerline song" every time they wander off it. I find my lack of ability to maintain a tune is a remarkably effective tool that quickly gets my students to magnetize the aircraft nosewheel to the centerline.
Feel free to torture yourself by listening to the song here... Or just practice mentally gluing your nosewheel to that nice thick white (or yellow) center stripe.
It is important to note that, although these lines are painted as a guide to pilots to keep your wingtips safe from obstacles, staying on the line will not always achieve this. As the pilot you must be situationally aware, not only of what is going on inside and outside your cockpit, but also of your wingspan.
At our home field we had a pilot with zero situational awareness blissfully following the taxi line, take out the fuel installation with his wingtip. Yes the guys who painted the lines should have been supervised better, but the final responsibility falls on the PILOTs shoulders. You are the brains of the operation, the aircraft you are operating is relying on YOUR brain to keep it safe. Protect your aircraft. Aircraft have feelings too.