Here's a few tidbits that I've brought from the "ergonomics" world of computing and applied to my copywriting with decent results and some rationale for why it works.

First, a note of warning regarding the online copy "frame" -- keeping with Oglivy's "Move away from what the eye is used to and you stop readership"

I've found that it's best to use an ivory background as opposed to a bright/stark white. Testing has shown a 13% improvement with this change alone. Rationale: Aside from the "used to it" factor, the harsh contrast of the black type on the unnaturally white paper or screen is just too hard to read for 66 pages worth of involved copy.

Additionally, Bob Stone (of Stone & Adler fame -- check out his book: Successful Direct Marketing Methods) shows that indented paragraphs USUALLY outperform those that with the more "modern", non-indented paragraph style.

So, you're mileage will obviously vary on this one. I haven't split tested this yet, as I've been honing other aspects of my craft. But it's part of the "formula" I use.