I first came to DFW through his Early Harper's non-fiction pieces, especially "Shipping Out" (about taking a cruise) and "Ticket to the Fair" (about attending the Illinois state fair).* These are full of both his humor and his human-ness. They are also the easiest intro's by far (I believe) if you are trying to introduce someone else to DFW without forcing them into, say, joining you in the "Infinite Summer" project. I'd probably alternate between his nonfiction and fiction books from that point on.
*Both of these articles appear in longer form in "A Supposedly Fun Thing I'll Never Do Again" (as the title essay and as "Getting Away from Already Being Pretty Much Away from It All"), but I actually recommend reading them in their original form first (links are below): While I usually have little sympathy for the "DFW needed more editing/cutting" arguments, I think the Harper's versions are crisper in their effect, even though I also enjoyed eventually reading the longer versions and laughing at or being touched by details I realized were not in the original versions.
Shipping Out:
http://harpers.org/media/pdf/dfw/Harper ... 007859.pdfTicket to the Fair:
http://harpers.org/media/pdf/dfw/Harper ... 001729.pdf