The fantastic clearfix, as a LESS CSS class:

.clearfix {
    zoom: 1;
    &:before, &:after {
        content: "";
        display: table;
    }
    &:after {
        clear: both;
    }
}

Use in your code like so:

ul#gallery {
    .clearfix;
    li {
        float: left;
    }
}

Easy.