Staying inside the lines is not a requirement with abstract art. In fact, straying off a line opens new opportunities, is this now a flourish that can be expanded upon? Have you created a new shading or blending opportunity? An oops is just an opportunity to explore. There is only one situation in which keeping between the lines is critical: driving. Straying from the line on the page does not qualify as a mistake, in abstract art there are no mistakes, only opportunities to add your own flourish or background blending. But if you must stay between the lines, hold your pencil straight up and down, and just like with driving, slow down and do not take your eye off the line.