Disclaimer: I don't know much about voice acting/recording other than the fact that it's harder than it sounds. Hopefully these will be helpful...
1. Don't narrate. Converse. Pretend you're talking to somebody who is listening attentively. Think of radio disc jockeys, who have to talk in a one-sided conversation all day long. They do their job well with the knowledge that they've got an audience on the other side of the wavelength. (I think.

2. Get comfortable, because a comfortable voice is exactly what I'd like to hear. After all, the guy is sitting there in his favorite armchair, having a smoke with his favorite pipe.

3. Feel free to make minor changes in word choice to make the passages more accessible to speech. I'm actually going to go back and revise some of these... I think... with permission... ?
4. Record each commentary in segments, or pause a bit between paragraphs. That's what Songbird did for her recordings. It gives you a chance to keep all the parts that you don't mess up on, and do multiple takes on only what is necessary. Run them all together when you're done though, to make sure there isn't a major difference in tone or something.
If anybody else has suggestions, feel free to post them.