For me worked best to file or saw off the "old" shoulders and sculpt new ones from scrap.
I used milliput (the white one, it hink its the "fine" one) for sculpting these parts as for mantles.
I like the material a bit more than the stickier other stuffs.
Further itīs in my opinion better workable after drying. I normally use scalpels for cutting the rough forms and then an assortment of files to smooth the surfaces.
If you get used to it a bit you have nearly nothing to work after drying if you get a bit used to smooth while moist with water and a fine brush.
Another easy way to get supersmooth surfaces is to use superglue (i use pretty cheap ones for about a Dollar per 20 ml) as a laquer.
For smoothing holes or other things or for sculpting transitions i use "magic-sculpt". Donīt know where its from but itīs a resin material and has the bonus of being pretty lightweight.
So you could use it for greater sculpts more easy.
Last but not least I like using the plastic Bauhausers or Imperials for a harvest for bits .....
They are pretty cheap and I cut off what I need, resculpt it and use it for modelling it to the new models ( for that I use mostly resin materials (magic-sculpt)).
Gives a good supply of rifles too.
I did a squad of Strike Kommandos this way. Iīm not sure atm if I already upped the pics of them, but have a look at the "conversion showoff" section. Some of my models can be found there as in the army-gallery section "Bauhaus".
Just looked it up, you can find the Strike Kommandos at my photobucket page, the small pics in the conversoin showoff section leads to it)
If you like anything you find and have a question about how I did it, feel free to ask ;-)
Hope it helps a bit