I don't tend to do much loose work in my illustration. Most of it is pencilled quite tightly so I know where I'm going with a piece and that usually helps keep the client happy too as they don't get too many big surprises heaped on them after approving the pencil work. I do like to experiment however and that usually happens when I'm colouring an illustration on a computer, painting traditionally or sketching.
I've become very fond of a Pentel Brush Pen I use for doing very quick inked sketches, see this post for an example of this. I take a much more measured approach to my inking for finished pieces but for a loose inked sketch I love using this pen. Its very fluid and gives some lovely surprises when you are relaxed and playing around with it.
I've started doing some quick inked sketches that will be given away with my comic 'One Last Time' at Thought Bubble comic convention in Leeds. I've blocked in the rough information with a blue lead pencil and then have just gone straight to inking the sketch with the brush pen. I think this first one took a couple of minutes pencilling and then about ten minutes to ink so you can see its very quick and loose from my usual approach but I thought that a free sketch with my comic would help the sales, maybe?
I'll typically stick to a subject I like and also mostly just portrait (head and shoulders) shots of a character. To start I'll be doing Doctor Who sketches and to make it easy for me I'll begin with just the Doctor, well he has regenerated a few times!