Well... Right now, my runtimes are in complete disarray, so I shouldn't pipe up here, but I won't let that stop me. So take this as lessons learned the hard way (and I haven't graduated yet).

I think before you begin organizing you should evaluate the types of work you tend towards, your work style, volume of products and accessibility.

One of the things that always frustrated me, is that while the multiple runtime iidea is good in essence, it was proving problematic with just about every conversion type utility I use. It's all very well to put all of V3 things in a V3 runtime, but when you want to use Sydney with V3 clothing, but use A3 hair you're still ending up spending a lot of time switching runtimes. Also, you end up with quite a bit of duplication if clothing shares texture maps across runtimes.

If you're on a Windows PC platform, the best advice I can offer is not to underestimate the power of the shortcut. For instance, I keep all my hair and texture maps in the largest runtime, because most of the content is "communal". I drop all of the mat files for hair textures, fits and poses in one subfolder of my pose library file on that runtime. The subfolder is divided by vendor, and then the particular hair. Then I drop a shortcut to that folder in all of my other people oriented runtimes Pose folder. I drop a shortcut to the entire hair folder (in the largest communal runtime) into my smaller people-oriented runtimes. 

I repeat this process with all light files, pose files, expression files, hand pose files and materials files -- though I keep these in the main Poser Runtime. Why? Because some of these items are python dependent and python files, as far as I know, have to be located in the runtime with the executable.

My runtimes span multiple drives, it can take two to three minutes to change runtimes, but with a shortcut folder, I can't even tell I'm switching to content in another runtime, let alone on another drive.

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