Corsets made of prims have a great advantage. They usually make your body stiff. By doing that, most corsets also make the left pec and right pec attachment point stiff. - Those points usually move really ugly and are hardly usable for anything, so it is likely you have them vacant.
I do recommend to add add the MoDesign Attach Anywhere script to the corset before you proceed. You don't need to do that with MoDesign Corsets, as Attach Anywhere is already installed with MoDesign Corsets:
- Rez your corset on some safe ground
- Drag the Attach Anywhere script from your inventory onto the corset
- Take the corset back to your inventory
- Wear your corset at the usual point
- Detach your corset
- Right click the corset in your inventory and choose:
Attach To -> left pec (or right pc)
-> Your corset will rez with some decent offset. - Edit the position of your corset
The use of a posing stand might be helpfull
When sattisfied: - Detach your corset and wear it again at the same attachment point
This is to save the position