When I was specializing in storyboards I used to send out a lot of promotional material. I preferred this method rather than sending things over the internet because the art directors would tack these pieces on their walls. I simply got more exposure that way as other art directors in the agency would see my promo pieces. The art directors would also have examples to show a client or the Creative Director if necessary.
My renderings were all done using markers because of the tough deadlines, this was the fastest method for me to use. I enjoy painting watercolors and rendering with markers is quite similar. Also, many art directors like the clean, fresh look of a marker sketch.
This piece promoted the fact that I could deliver comps, storyboards and even Animatics over the internet.
Sending them an impressive client list is also important, it gives one a lot of credibility and reassures them that you're a professional that can handle tough deadlines.