1.
Start with lines.
Most artists begin a drawing with a kind of shorthand, a series of gesture lines or construction lines -- just something to get them started and to map the placement of the major shapes.
I'm comfortable using the marker itself to make those first lines.
However, if you find such lines to be too obvious or intrusive, try using pencil for that initial line-work.
2.
Make clear shapes.
Areas of light surrounded by half-tones will "read" if they are clear and definite.
3.
Control your values.
When you touch a marker to your paper, it makes a positive black mark that can't be erased or altered.
To make a gray tone, then, you have to make a pattern of black marks mixed with the white of the paper: scribbles, dots, or the simplest, line-groups.
All the halftones in my drawing are grays produced with line-groups.
You will quickly get a sense of what a marker can do if you fill a few pages with scales or shapes of different grays.
Try dragging the side of the marker, as well as the point, and try varying the pressure of your strokes.
4.
Use a slip-sheet.
If you apply the marker heavily, or in layers, the ink will bleed through your light sketch paper and make blots on the following page.
 It doesn't bother me, but if you prefer to keep your pages clean, put an extra sheet of paper behind the page you're working on to soak up the blots.
The one sheet can be used over and over, of course.
5.
Save those old markers! Don't throw your markers away as soon as they start to dry up.
You can use a marker that's running out of ink for quite a long time, to make an effective "dry-brush" for light tones.
You can make gray tones by dragging a semi-dry bullet-point marker on its side.