This is a quick demonstration of what I believe is the best way of getting some fluid controlled lines using a Wacom tablet. The technique is very simple, and pretty much anyone should be able to get good results with a little practice.
The main things to bear i mind are that it is much easier to draw a long curve or line if you think of your arm as a compass, with your elbow as the pivot point. Being left-handed I have a preference for drawing curves in this direction:
However most right-handed people will prefer to draw a curve in the opposite direction. If you've been struggling to get smooth controlled strokes when using a Wacom tablet, it's worth bearing this fact in mind - because the tablet is placed on the desk, and isn't as easy to turn as a sheet of paper can be, it's easy to overlook the most comfortable way to draw. And by trying to draw the hard way (against your natural direction), you could end up struggling to get good results.
When I need to draw a long line, either straight or curved, I lock my wrist so that it doesn't move and pivot my hand and forearm from the elbow to create the stroke. The movie below demonstrates this motion, and the overlaid lines are to show that the forearm and hand are stiff and do not move while the arm traces the desired arc across the tablet:
This technique enables me to draw long curves and straight lines going down the tablet from top to bottom. This motion is the one that I personally find the most comfortable and easiest to control, especially over long distances.
When sketching I generally work loosely and am happy to draw from the wrist without flipping or rotating the image. This can lead to some shaky lines but it's not a problem at this stage. For finished work however, control is a necessity. I usually sketch in any one of Photoshop, Sketchbook Pro or Painter, for finished linework though I will only use Painter - it offers a lot more control.
Painter offers two enormous advantages over any other program for drawing lines: it enables preview rotation of your image (without actually rotating the file itself), and controllable damping of the stylus. This means that you can rotate the canvas much as you would a sheet of paper, to get the angle that is most comfortable to draw, and that you can control how smooth the lines will come out.
The damping control is found on the brushes palette in Painter, set it to 50% for most lines, but for more difficult longer curves you can set it higher for a smoother result:
To show how this process works here is a quick step-by-step, inking a sword which was picked as a subject because of the long lines required to render it.
Firstly, a sketch done in Sketchbook Pro to establish the shapes, this is done within the limitations of that package which only allows page rotation by 90º increments, enough to get by in this case:
I then take it in to Painter and select the Scratchboard Tool which gives a nice thick-and-thin line. To draw each curve I will rotate the page to the most comfortable angle and allow my lines to overshoot the start and endpoints if need be.
Each new curve can be created on a new layer and then merged down, so you can concentrate on each line one at a time. Once the line is drawn the ends are erased back to conform with the sketch, and using the eraser I also taper the ends.
I draw the rest of the lines rotating the page where necessary:
Then when I come to the bottom of the blade, I flip the canvas so that the curve will conform to my preferred direction, rotate the canvas and draw the line on a new layer, and erase the ends and taper the line as before:
Although sometimes the lines may need a couple of attempts (after an undo), overall this process is very quick and extremely easy to control once you have a little practice. Trying to draw long curves or lines in uncomfortable positions, or using your wrist as a pivot point over a long distance, are exercises in frustration. By using the correct technique you should get great results very quickly.
Once you are comfortable with long lines, you can also learn to draw ellipses from the elbow - this requires lifting the elbow off the desk so that it can move along the arc of the ellipse, whilst still acting as a pivot point, and keeping the wrist stiff.
If you want to have a practice along with the tutorial try saving the original sketch of the sword from this page and inking it yourself in Painter.