I have to leave for a few hours, but in the mean time you may want to review these video tutorials Phil Cooke published on Poser's joint parameters:

http://www.youtube.com/watch?v=PfcPu83gZNs

If you rigged the conformer using Nearme's own bones (the "donor" approach) then the falloff parameters really ought to already be done, but you'll want to understand this just the same.

I think part of your problem is going to be that you're bending the thigh, and because of the way the pants are rigged and grouped, you will see an abrupt break where the buttock group joins the hip group.  This is a weak point of Poser's group-based rigging, you can make a JCM for it but that's about it.  If you're used to rigging in Maya you probably won't be too happy to learn this.  Creating a JCM in a conformer by hand is a pretty complicated process involving editing the CR2.  I don't know of a tutorial that covers this process.

If the character is actually rigged hip -> thigh then you may be able to fix this with joint falloff zone adjustments, see the video tutorial.

edit: or as wdupre suggests you can also try changing the way the buttock is grouped on the conformer, yeah, but in my experience it can be pretty tough to find a compromise.

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