This has been a great thread that has given me a lot of useful ideas, so I'd like to get it going again. I also hope that my own experience might prove helpful to others considering the switch from 4 to 5 for the first time.
My first experiment with a fiver was a dismal failure, one made doubly frustrating by the fact that it was an incredibly cool bass with amazing tone -- a Reverend 5L -- that I just loved in every way other than my total inability to play it. This was before the present thread, but I had figured out on my own two of the transition strategies since discussed here. First, I was moving across country to live on the other coast for a year, and made a conscious decision to bring only the recently-acquired fiver with me to force me to focus on it exclusively. I also decided to start out trying to think of it as a regular 4-string, using the low B string almost exclusively as a thumbrest until I got comfortable with. I never did. I finally gave up and traded it for another four-string (making fellow TBer Trevorus very happy). It was just too frustrating and I wasn't having any fun.
I am convinced, in retrospect, that the problem was this: Switching to this new bass involved not just one, but FOUR major changes, and I simply couldn't adjust to them all at the same time. In addition to having the "extra string" -- and thus frequently confusing the E for the B, etc. -- the Reverend also had (1) a fatter neck, (2) a longer (35") scale, and (3) narrower string spacing than any of my fours. The combination of the latter two was especially problematic, because notes were now both further apart horizontally but closer together vertically on the neck. Consequently, the strategy of pretending it was a regular four-string with a very long, skinny thumbrest simply didn't work. I think that if I had switched to another four-string with a longer scale, fatter neck, and narrower string spacing, I would have had almost as much trouble even without having an extra string to worry about. So that's my theory, and I'm sticking with it.
I just recently decided to put this theory to the test, and bought another five-string: this time, an MTD Kingston Artist (thanks, pharaohamps!). I picked this particular model because it has (1) full 19 mm string spacing at the bridge, and (2) a very thin and asymmetrical neck. In stark contrast to the experience I had when I first wrapped my hands around the Reverend (basically, "***??!!"), this thing felt immediately comfortable and wonderfully familiar. The neck profile and wide string spacing make it feel almost exactly like a four-string jazz bass (but, of course, with a very long, skinny thumbrest). The extra inch of scale length seems barely noticeable given the familiarity of everything else. I can tell already that it is going to be WAY easier to learn my way around a five-string with this baby. I'll post a progress report in a few weeks and let y'all know how the theory stands up to the data.