One way would be with CSS that adds margin-bottom to col-* on smaller devices/widths..
@media (max-width: 768px) {
[class*="col-"] {
margin-bottom: 15px;
}
}
Another way is to add a div that is only visible on smaller devices/widths..
<div class="col-sm-6">
<div class="hidden-lg hidden-md hidden-sm"> </div>
...
Demo: http://bootply.com/92276
Update 2019
Bootstrap 4 now has spacing utilities that can be used.
Also see: Add spacing between vertically stacked columns in Bootstrap 4