ok, for rigging and poser details, Nerd 3D has loads and loads of invaluable information. Most of what I learned is by sticking my head up his arse and begging!  You can see he has a lot of published info here: http://www.nerd3d.com/

Another thing I find invaluable are Dimension3D's poser tools. Poser File editor in particular. I would die without it: http://www.renderosity.com/mod/bcs/poser-file-editor-3/62322/     D3D has few other amazing tools, here at Rendo and at DAZ (for DS/Genesis content)

I'm not sure what you mean by adapting pre existing textures? For characters, if you use pre existing, make sure they are licensed as 'merchant resource', or photographs meant for texturing, like what you can get from www.3d.sk. You can't take other textured characters and tweak the images (for freebies or for store items). For your own personal use, you can of course do whatever you want.  Promo images are very important of course, spending some time learning how to make a cool catchy render is smart. Look at advertising outside poserdom, fashion, movies, games... whatever may be somewhat similar to the content you are making, That is a learning curve of it's own. Whenever you can afford, hire a promo artist. I do it about half the times, especially for the main promo. You get only a second or two for the buyer to fall in love with your product - it is only after that they most will bother to read techy details - especially until you develop a following and a name recognition. And, well, you have to make sure that the product stands up to the promos, or people will feel like they've been cheated... and poser users can be very vociferous when they don't like something.

Fancy lettering - first, don't overdo it, learn a bit about graphics design and visual hierarchy of things so you don't assault the customer with everything at once and they run off with a visual migraine. ok, ok, i'm exaggerating a bit....   lot of little designs are a combo between layer effects, and whatever fonts you can find out there on the internet, there are tons out there free or for pay (check EULA's on everything, make sure you are allowed to use it in a way you want to use it.) You dont want to get in a hot water copyright wise, not only because of the other party, but if it happens more often, the brokerage starts being nervous about taking your product. Usually when there is a copyright violation they are the first to get the notice.

Thanks for this.

I bought the file editor in November. Haven't looked at it yet. Hope it helps.

Yes I do mean use a merchant resource. This is of course to avoid copyright issues and to speed the overall process of figure creation