Pen and Ink
I began sketching with pens in college. I prefer ink, because pencils and erasers slow you down! If a line isn't quite right, erasing it isn't the answer, because once it is erased chances are that when you try again, you'll make the same mistake. You are wasting valuable learning time.
Pens force you out of perfectionist habits. When you work with a pen and your line goes all wonky, the only thing you can do is embrace it and work with what you’ve got. Allowing yourself to make mistakes helps you become less precious about your drawings. I don’t believe in drawing mistakes. If you keep going, your drawing will become something new with each line you add.
Here’s another way to think about it: when you’re walking, you’re not looking at your feet — or even behind you — to see if you took the right steps. You’re just putting one foot in front of the other to keep moving.