I never rely on client or venue markers because, irrespective of your medium, you will generally be provided with whiteboard markers.  Usually bullet tipped, old, dried up, and faded.  Even if they are brand new out of the box, they won't last ten words on paper because paper is porous and soaks up the ink. Great for whiteboards but not flip chart paper.

So you may be asking, “what is a bullet tip?”  Bullet tips are rounded and create only one type of line – usually thin. 

What we want for great flip charting, is a chisel tip pen.  They are squared off or angled and give you many different line types.

My favourite chisel tip markers for flip charting are Neuland, Mr Sketch or Charters Markers (from The Grove).

Three things:

1. Don't rely on client or venue markers if you are planning to do flipcharts for your sessions.
2. Invest in your own kit.  It makes you look more professional.
3. OBSESS about your markers, they are an important tool of your trade.