This was written before kuroyume's response,(he really knows his stuff) but I thought I would go ahead and add it anyway. 

Load both the figure and your clothing.  Conform cloth. Setup your figure so that it is in a pose that causes those problems in the cloth fitting.  In the window, have the cloth and a body part selected; lets just say rCollar.  Now open the "Window/Joint Editor". 

In the "Joint Editor" palette, you should see a tic mark and the word "Center".  When you click on this tic mark, you will find the parameters that control the twist, up-down, and side-side movements.  Click on one (make sure the little "Display Deformer" box is checked).  Let's pick Up-Down; This could also be the Y-axis.  This sort of depends on how your joints were setup to begin with.  You should see some green and red arms coming out from the collar.  You may also see green and red circles; these are the "Sperical Falloff Zones".  These will only be here if the cloth was setup to have them.  Without getting into alot of mumbo jumbo, which I don't understand anyway, you can make adjustments on those green and red arms, and those green and red circles.  Either by direct selection of the arms, or through the "Parameter's Palette".  

More than likely, and I'm just guessing here; you need to scale up, ever so slightly, your Sperical Falloff Zones.  With the cloth and rCollar selected, and those green and red circles showing, click on the tic mark that says right collar in the diplay window.  When it opens a little box, pick props and then pick "inner Mat Sphere".  In the "Parameter's Palette",  scale it up while looking for any changes in the cloth's positioning. 

Most of this, for me anyway, is just a matter of trial and error.  At some point in all these tiny adjustments, I get what I want, then I apply the symmetry command, reparent the cloth to the Universe, and then resave it to the library.  This process may have to be repeated again and again before I'm really satisfied that it conforms properly in most every pose I throw at it.

I hope this provides some help.  I'm not vey good at explaining things.  I highly recommend the book by B.L. Render: "Secrets of Figure Creation".  I know it's not cheap, but it was worth every penny for me.  It really made me understand how to make figures work in Poser.