Pencils have the advantage if you spill your coffee, or walk out to the aeroplane and it raining.

Personally I use a twist-propelling chinagraph. They're very easy to wipe off using the cuff of your flying suit (of another suitable piece of fabric) once the information is no longer required.

The other option is to use two layers of plastic on the knee board, then you can write any details that won't be changing during the flight using pen on the bottom layer, which will be protected by the top layer, upon which temporary info can be written (eg pressure settings, info codes, Rwy/circuit details etc) in chinagraph or non permanent marker...

The other top tip - tie the pencil to the kneeboard! FOD is the last thing you want!