I use the refsworld pencil, as
@Ross suggest, but only as backup. I find it a bit too small to grab in the heat of the moment. I have bought pencil clips like these
http://www.ebay.com/itm/Twelve-Faultless-Pen-Pencil-Pocket-Clips-Dclips-/151571570958
I then buy a 4B pencil, saw off 5 cm and then sharpen it. That way I have a custom fit for my ref kit pencil with the softness (4B) I have found to be optimal. (I also tried with 2B, but found 4B to be better)
As you can see, I really bring a certain nerdiness to my equipment