1) 5 min. live-model-sketches can be stressful!
2) So, better think ahead, which art tool you prefer to use today and not switch from one to another at the session.
3) It really does make a difference thinking about composition, thinking about the space that is used up by the character and the space around.
4) You are at an advantage having reappearing models, because you get to know them by observing.
5) Which can lead to a more loose approach, not worrying so much about proportion or likeliness.
6) Which, in turn, might give you the freedom to let your imagination run wild... or, at least, letting it bounce around a bit.
There is a very interesting book about drawing characters from life. It is called 'The Drawing Club' from Bob Kato (link at the bottom). Just got a couple of pages into, so I will only say: You want to learn from some of todays masters of illustration and animation, you want to inhale those tips and exercises and, of course, enjoy the great drawings! ... from the book, that is.
THE DRAWING CLUB by Bob Kato - Link to Amazon
Also check out: First Live-Drawing-Session... Ever!