Talking: What Do I Do?

I have noticed that with my 6th graders if I say, "this assignment is individual, please do not talk." They will always end up talking and I will always end up shooshing them.  However, if I tell my students, "you may work together on this assignment, please share your ideas,"  they rarely, if ever talk.  So what do I do?  If  I say they cannot talk when I want them to talk they are going to think that I am not true to my word or that it is okay to talk at inappropriate times.  What should I do?