I just did a cursory read through this thread, but I am surprised no one mentioned the MOST important accessory of all - the incident light meter. Knowing how to use one with natural light will make or break your image. It's particularly important if you're using it in combination with a flash - like in a single light set up environmental portraits or even with your on camera flash or hotshoe mounted flash. It's also particularly useful to know just how much fill light you'll get with various available reflectors.
I 'meter' the general environment so that I know the light in the nooks and crannies, by the various windows, etc... and not having to meter before every single shot. Anyway, this is an old thread, so I'm not sure this information is useful to you any more. Show us some pics if you've taken them already.