I think these days artists are under a lot of pressure. If you learn how to simply distance yourself a little from your work and not take how it turns out to heart, it's easier to loose the stress.
How about you buy a giant butcher's paper pad get out a big black permanent marker and just got to town and draw anything you want, however you want. You can throw out, burn keep, do whatever you want to the work after.
Your style and work is just like your face, you have to work with what you've got and embrace it! I find the worst thing possible is to be generic and run of the mill. Messy art is always more interesting and gets more notoriety.
Drawing neat, confident lines comes with practice. Perhaps it may come quickly for some people (due to steadier hands or previous experience with fine movements, etc), but as long as you keep at it, your lines will become steadier and steadier. It's normal for hands to tremble with finer movements at first and it's just muscle memory in the end. Your hands will know what to expect after a while, in a way.
Messy lines are not that bad anyway in sketches hehe! Just practice and practice if you want some neat lines Practice your hand by drawing random lines... circles and stuff with Pencils first (to erase the mistakes) and Pens later (to make it harder without erasing)
From what I've seen (and experienced), it's a common problem among beginners. When I started out my lines were really scratchy-looking (and they still are in areas where I'm drawing something I'm unsure of). With practice, my lines have become much smoother. It helps to practice drawing loosely from the shoulder as opposed to from the wrist. As others have said, the more you practice, the more your confidence will build up and you'll be able to get the line you want in a single, sure stroke.
Thanks for your responses folks. I definitely would like to be able to draw freehand curves and it's something I'm spending a lot of time practicing... Just drawing curves, letters, numbers, circles etc
So is the verdict that this is mostly a problem of inexperience and that even with my somewhat shakey hands I should be able to draw good lines if I keep practicing?
You could try drawing at different times of the day. Your body may be more relaxed late in the evening. Also, do you drink coffee or energy drinks? Both the other commenters are correct though, the only way to get a better freehand line is to practice. If you don't really care about having the ability yourself and just want smooth lines in your work, you could always use a french curve
If you consider your lines messy and you don't like them, your options are to work with what you have and consider the lines in your final composition or work on them to make them cleaner. There are plenty of artists who have a messy look to their final pieces. As you practice on drawing clean lines, it should get better with time. Another option that might help you is using a ruler, or a guide line to get straight lines . I know that a lot of times, even now I need an outside tool for help. In my opinion drawing perfectly straight lines and perfect shapes is actually a difficult thing to do! I hope this helps.