Here's a small collection of useful drawing tips when wireframing or doing low fidelity prototypes. Some of them may seem obvious to some folks, but I think they are all worth mentioning.
Add additional length to paper
Don't let the end of the page keep you from moving on. Tape (or staple or glue) additional paper where you need it. This is generally towards the bottom of the paper. but I don't see any reason you can add it wherever you need it. Add one piece, or two or three. The idea is to keep moving. You can also try elongating the paper even before you start sketching.
Use color to make notes and differentiate functionality
UX designers need to be visual communicators. By using different colors, we can delineate aspects of the design, make it more organized, and most importantly clearly communicate our intentions. Keep a collection of different colors close by. I like using colored pens, but colored pencils have their place too.
Use Copic markers to add contrast to elements and modules
Copic markers may be on the pricy side, but they are well worth the investment. They last a long time. They are very high quality, making an even tone that is consistent. Copics have been used by architects and industrial designers for years and are a great way to add contrast and tone to a sketch.
Focus on alignment and organization
Using a light pencil stoke, or a ruler– loosely mark how you want elements to line up, then draw your element using those guidelines. All design software has guides you can pull to keep alignment in check. This is the same concept, only with pencil. Get in the habit of drawing guidelines, it will end up making your drawings more clear and organized.
Communicate your idea visually.
Sketches are ideas, they are there to serve as a concept for what will eventually become something. The more you can document, make notes, and describe the idea, the more of a chance that idea has to become a reality. Copy writing, developer notes, user perspective, responding to a boss's concerns. Whatever it is, shape it and mold it on paper.