Do you mean hit undo when you are painting? Or do you mean when you are animating?
A couple of things to check:
Make sure when you do the Soft Bind you delete the history on the model and that all transforms are frozen.
If you are painting weights make sure you have it set to Post not Interactive.
Make sure the mesh you are skinning is a model that is not in layers of geometry. That it is basically one mesh. Looks kinda like you have a body layer and clothing layer. ( I don’t mean display layers I mean overlapping geo)
If you have overlapping geo then you’ll want to do painting. Also will be better to simply model it so that it is one piece. If you are going to do cloth dynamics you’ll want them as separate objects.
And finally there are some recent bug fixes with smooth bind and painting. Check out the list of bug fixes for the current release if you can find it and if you have a student license you won’t get those till 2015 release.