Focus on the details that matter the most to you. On somedays it might be a nose, a forehead, a hand, some shoes, or a color scheme. Often it’s little bits of light.
Overtime, if you look back at your quick studies you’ll see, staring back at you, what matters most to you as an artist. Armed with that awareness you can reassess your goals, reset your intention, and start working on the skills that will make it possible for you to achieve those goals.
The great thing about this approach is that as a quick study you’ll also be working on your visual memory and your speed. Even if you happen not to be working from a live model one day, you’ll still hone those skills for the next outing.
Try doing this over the weekend, morning and afternoon. On Sunday night check your sketches (at least 4) and ask yourself where they are pointing you. What new skills do you need to acquire? What skills do you need to improve? Where are you most likely to find subject matter that will help you with your goals? What tools do you need to be working with to work quickly efficiently and capture the most detail? What tools are slowing you down?
Is your sketchbook sized appropriately for how you work? (By that I mean, if you stand, is it not too large that you can hold it? If you sit will it be so large it takes up a whole restaurant table?)
Make strategic decisions to make your quick sketching an effective part of your drawing practice.