I'm sure that this has been asked somewhere here before, but how do I get a texture like denim to stick to a shirt in P5?

If you have a denim texture in a jpg, then click on the little plug next to the "diffuse color" channel and select a 2D image map as shown above.

Next, click on the "none" in the image node box, then browse to the location of the texture.

Now your texture is applied to the first material group in the list. In order to apply it to all others...

...right click anywhere in the materials window and choose "Select all". Then right click again and choose "Apply to all". Poser will you if you're sure - say yes, and voila. Warning - applying to all will only work for items which use the same texture for every material group. Using it for Vicky will give you solid eyelashes, etc. After clicking "Select all" you can also select "copy" by right-clicking, and then choose the different material groups and right-click to paste in the right ones. Hope that helps :)