You may want to look on Poser communitites for skin texturing tutorials, renderosity.com probably has a few in their tutorial section, and possibly daz3d.com may have some tidbits as well. Old Poser tutorials will probably be of more use, those designed with Poser 4 in mind, because more shading was required just as it's required for SL.
http://www.students.yorku.ca/~harnock/tutorials/skin.htm This is a really great tutorial for a very basic skin texture, the concept would make a nice base to work with.
I've recently created a set of skins for the first time for SL, although I've created several for Poser figures, and the process is much the same althougth the template is a bit odd in places - namely the hands, FEET, and face. So extra attention was spent tediously perfecting those areas that are so different than most UVs.
In a nutshell the process is to create a rough base, then add either detailed handpainting or photo-realism by using reference photos such as those from 3d.sk. The key areas to detail with photos would be the face, hands, knees, and chest, beyond that it would be up to you whether to add detail with shading or photos. I found that painting shading and highlights gave me much more control, for areas like shoulder blades, the collar bone and neck, the chest and abadomen..the toosh etc. I used my tablet for control over the shading, with a mouse it would be a bit more difficult, with a mouse I'd suggest to paint with one consistant opacity and then erase to create gradual shading. Gaussian blur could work also, but it may cause seam problems..
Then of course there are the seams, I'd recommend just not painting outside of the base within a 3-5 pixel radius of the edge of the seams, there should be little detail or variation around the seams so that they aren't an issue in SL. If through your process they become an issue, I recommend going around the seams with a solid coloured soft brush that's 5 pixels or so larger than the seams on all the maps.
Also I should mention if you need to create more than one skintone, it's good to use a neutral tone to work with, then (once finished) if you have Photoshop (not sure if other programs have this feature) you can use variations (Image > Adjustments > Variations) to create different skintones. Each time you make a new skintone you should save the preference, and before you make a different one be sure to click your original so you have a clean slate. This retains your shading and colouration much better than using Hue/Saturation.
I hope that gives you a little insight. ;3