Product packaging, advertisements and receipts are great drawing subjects. Focusing on small parts of a design, you can learn to better control the lines you make. This page was drawn with ultra-thin Sharpie permanent markers on Drawing paper. A mix of logos, font designs, tidbits from books and boxes, and doodles. You can do this anywhere - just pull out what you have with you - receipts, books, gum, postage stamps, pens, the sticker on an apple, a packet of sugar. But instead of drawing the entire packet of sugar, concentrate on the text, logo or other graphics. The numbers in the UPC code on a book are different from the numbers on a Lipton tea box. Notice how letters align with adjacent letters and try to imitate the look as closely as you can.
This is good practice in controlling where to start and end a letter, how wide, how much space in between, lettering in a curve, etc. Note the relationship of characters with other design elements. I'm not sure if this serves to strengthen the muscles you use to draw, or is just good experience, but it definitely works. Have fun with it!
More ideas for practicing and experimenting with pen and ink: