Apply the highest possible smoothing for the tool you are using. Start drawing a line, then hold for a second. The drawn line will become perfectly straight, and you can continue drawing the line by pulling the end out to the desired length.
Keep the original line short for a straight line. If you draw your original line too long, and/or with no smoothing enabled, you may end up with a curve that will stretch to the length you pull the end to. This can create some interesting shapes as well.