well!! good question!!!!
i think you will get lots of different answers, people that have been using poser for a long time might have some other ways of doing things so ill just give you my observations and experiences!
FaceOff has the best tools that i have seen out there for poser! i would suggest getting at least one of his tools, you can go with his SSS tutorial but thats quite involved! ibl lights make a big difference in any render you want realistic, combine that with some SSS and you are bound to end up with a winner!! but i will say that a good light setup is the best way to get a realistic render even without any tool! but the basic building block before all these is most sertainly a good texture! pull out your wallet and buy a good one! look at morris in the store or shahara! in my opinion they deliver the best textures out there! or wait! even a better idea if you dont want to spend money yet!
check on as shanims website in the freebie section he has a texture that is awesome! Luna is the name! tru it!

hope this helps!!!!