Developing the discipline to spend some time drawing every day is a key factor in the growth of artistic skills. Make an effort to spend at least a few minutes a day sketching, even if it's a doodle.
Working with a non-erasable medium is a great way to practice your drawing. Pens and markers are excellent for developing confidence. One of the things I try and get across to my students who tend to draw light and timidly with a pencil, is to switch to something else that will put a permanent pigmented line down right away. It'll open up new areas and expand your abilities.
I did these doodles in preparation for class. All in ball point pen, with the exception of the 2nd sketch from the left which is a marker rendering. Click on the image and you'll get a better picture.
Notice the difference between the two. Ball point pens are responsive to pressure and you can vary the sensitivity of the line, while markers are more direct. Spontaneous, free form drawing is a good habit to pick up, no matter what your medium. Try sketching with a pen next time and see what happens.