Fortunately Twitter Bootstrap arms us with an arsenal of classnames for components, grids and other UI-goodies. Unfortunately, they don't supply us with something as basic as layout helpers. Here's what I've used:

UI Helper Classnames

.margin-top {
  margin-top: 20px;
}
.margin-right {
  margin-right: 20px;
}
.margin-bottom {
  margin-bottom: 20px;
}
.margin-left {
  margin-left: 20px;
}
.margin-sides {
  margin-right: 20px;
  margin-left: 20px;
}
.margin-tops {
  margin-top: 20px;
  margin-bottom: 20px;
}
.margin-all {
  margin-right: 20px;
  margin-left: 20px;
  margin-top: 20px;
  margin-bottom: 20px;
}
.margin-top-none {
  margin-top: 0;
}
.margin-right-none {
  margin-right: 0;
}
.margin-bottom-none {
  margin-bottom: 0;
}
.margin-left-none {
  margin-left: 0;
}
.margin-sides-none {
  margin-right: 0;
  margin-left: 0;
}
.margin-tops-none {
  margin-top: 0;
  margin-bottom: 0;
}
.margin-none {
  margin-right: 0;
  margin-left: 0;
  margin-top: 0;
  margin-bottom: 0;
}
.padding-top {
  padding-top: 20px;
}
.padding-right {
  padding-right: 20px;
}
.padding-bottom {
  padding-bottom: 20px;
}
.padding-left {
  padding-left: 20px;
}
.padding-sides {
  padding-right: 20px;
  padding-left: 20px;
}
.padding-tops {
  padding-top: 20px;
  padding-bottom: 20px;
}
.padding-all {
  padding-right: 20px;
  padding-left: 20px;
  padding-top: 20px;
  padding-bottom: 20px;
}
.padding-top-none {
  padding-top: 0;
}
.padding-right-none {
  padding-right: 0;
}
.padding-bottom-none {
  padding-bottom: 0;
}
.padding-left-none {
  padding-left: 0;
}
.padding-sides-none {
  padding-right: 0;
  padding-left: 0;
}
.padding-tops-none {
  padding-top: 0;
  padding-bottom: 0;
}
.padding-none {
  padding-right: 0;
  padding-left: 0;
  padding-top: 0;
  padding-bottom: 0;
}