Once the lecture begins you'll want to begin your "circular breathing" of listening, synthesizing, and visualizing. It's important that you're able to take in what's being said while recording it, and not just stick your head down in your sketchbook. One of the most important assets is your "mental cache": the spot in your brain where you can store temporary ideas. With practice, you'll be able to store multiple quotes, thoughts, or ideas in a queue while you're sketchnoting. This "mental cache" also allows you to listen to multiple points and synthesize them down to what's important—before writing anything.
Inside of your sketchbook, you'll use a few key elements build your sketchnotes:
Text - Recording the verbal is quick, direct, and clear and is usually your primary sketchnoting tool. Capture the meaningful quotes and key points, and avoid trying to summarize everything. Typographic treatments can be used to give emphasis to major ideas, and can add interest to large blocks of text. Avoid making lists or outlines and use the spatial properties of the page to your advantage by "chunking" information. Some ways to force yourself to work spatially might be starting in the middle and working outwards or working in columns for a panel discussion.
Containers - Simply enclosing words in shapes brings emphasis and structure to an otherwise wild page. Some of the more common containers include (but are not limited to): quote bubbles, boxes, circles and thought clouds.
Connectors - Connect ideas and pieces of stories with arrows and lines. A basic chain of thoughts can scintillate around the page and still be clear if they are linked with a simple set of connectors.
(Thanks to Will Rice!)