I did not write this, but it's important to remember, so I'm copying this from the Renderosity Forum so I can find it easily. This was originally written by 3dcheapskate.
Select the figure you're trying to conform to, do Figure > Spawn Full Body Morph..., and give the morph a name starting with an exclamation mark (e.g. "!MyFBM"). A new morph will be added to the figure's set to zero. Don't touch it.
Load the clothing item and conform it, then do Figure > Copy Morphs From..., and select ONLY the FBM you just spawned (starting the name with an ! means it appears at the top of the list - that just makes it easy to find)
Dial the newly added "!MyFBM" morph on the BODY actor of the clothing to 1.0, and it should fit much better. Note: If the clothing has already picked up some super-conforming morphs from the figure you may have to dial those back to zero.
More details on the "Tips/Tricks/Tools for getting the best fit of clothes to the body - Suggestions and help with general ways of getting the best fit of clothing items to the bodies" thread over in CGbytes Poser forum, post #20 onwards - here's a link to post #21 of that thread http://www.cgbytes.com/community/forums.aspx?g=posts&t=115158&p=2#post125269