Watch the way you’re breathing; take a deep breath and pull the line as you exhale. Warm up exercises like drawing circles or shapes in perspective for half an hour to an hour can help you a lot too.
Also when you’re sketching, by thinking “I hope my hand doesn’t shake” or what have you might be the reason why you’re having this problem. Try to have a “zen” mindset when you’re drawing. For me, listening to music (something with no vocals and not on the “heavy” side; like classical, house, electronic, etc…) or a podcast can work greatly.
Hope this helps!