A sketch begins to go wrong even before the subject is put on the paper. As the pencil makes contact, your hand grows tense, and you rush to create the initial outline, which seems as though it must be the final contour. As your pencil presses more strongly against the paper at the very beginning, you are more likely to make permanent marks, and you might struggle to correct a small proportion error that you have made as you move on through the drawing. This is how the lines drawn by the start of a sketch is not designed to show whether you got it right or not. Their purpose is merely to facilitate your search.
Before you start drawing the subject itself, make a few warm-up marks on one corner of your page or a separate sheet of paper by drawing a few loose straight lines, loose curves, circles, and small squares. The purpose is to get your wrist used to the movement of your hand without requiring that it immediately draw a precise figure. As long as the marks remain light and appear incomplete, the sketching process has been set in motion.
When you do start your subject, resist your first urge to immediately darken the outer edge. Instead, try to think of your drawing as an entire unit. A cup should start as a loose cylinder, as should a book as a soft box, before you even draw the leaves in a plant. Start with some light construction lines, which indicate the height, the width, the tilt, and the major directions in the object. Do not be afraid to draw over, disregard, or alter these construction lines. You do not have to erase every pencil mark. You merely have to ensure that they remain subordinate to the drawing.
You can also associate the intensity of pressure you apply to a pencil with how urgently you want the object to look completed in your sketch. If you press harder against the paper in order to make a clean outline in an early stage, you could miss the chance to make observation. It could prove useful to step back after the first few construction lines to determine if you have gotten the right proportion. Can you tell if the top side is bigger than the bottom? Have you noticed if the right side is more tilted than you first expected? Are there gaps that you can use to check if the angles line up correctly in the subject? These comparisons become easier once the marks have been lightly made.
Try to make a few attempts at the same simple subject, to see which pencil pressure helps you draw the most. Draw the object in two separate sketches to compare their results. Spend the first minute or two on the first one, using light construction lines without adding hatching, heavy outlines, or surface details. Your sketch should just focus on its shape and positioning on your page. In the second sketch, do the same object, but use a combination of dark contour lines as you determine which ones are most appropriate. You can likely notice a difference that this drawing will feel a lot more relaxed once you have already determined where the final lines should be.
Darken the lines that matter, so they have the ability to convey different depths in your sketches. You might decide to draw heavy lines in the closest edge to you, to indicate the bottom of a shape, or to highlight a contour you want to pay special attention to. When all lines have been made with a similar intensity of pressure, such distinctions become lost. This is where light construction lines become helpful, as they can be easily darkened, or covered with more definitive contour lines, or even ignored. You will have more space available to make your lines clear and visible as well, which will help you refine any shadow shapes or hatching once they appear.
At this point, you should not think that a quick sketch has to look clean to be useful. Ask yourself, do the initial marks help me position the subject? Am I more aware of a single problem I have with drawing proportions? Are the lines I darken selected with thought rather than instinct? Your sketchbook page could still be a valuable exercise, with light construction lines that help you draw, even if it is not the most finished drawing you have ever made. You will see how your sketch improves with practice by recognizing when to make a few light marks to search the object, when to make them more visible to revise the drawing, and when to press harder to finalize it.