A little bit of SASS that lets you define a new breakpoint and will automatically generate a new grid for that breakpoint.

_responsive-grid-extender.scss

// As used by Compass.
// Used to calculate the em value for the media queries.
// If you're not using ems for your media queries, why the hell not?
$browser-default-font-size: 16px;

// Define your breakpoints by supplying a name and a pixel value. Units will get stripped so you can add 'px' for clarity if you prefer.
$breakpoints: false !default;
$breakpoints: small 480px, medium 768, large 960;

// Strip units from values so we can do maths and shit.
@function strip-unit($num) {
  @return $num / ($num * 0 + 1);
}

// Loop through each defined breakpoint to generate our grids.
@if $breakpoints {
  @each $breakpoint in $breakpoints {
  
    // Get the breakpoint details.
    $breakpoint-name: nth($breakpoint,1);
    $breakpoint-value: strip-unit(nth($breakpoint,2)) / strip-unit($browser-default-font-size);
  
    // Define your grid.
    @media (min-width: #{$breakpoint-value}em) {
      .grid-#{$breakpoint-name}-1of1 {
        width: 100%;
      }
      .grid-#{$breakpoint-name}-1of2 {
        width: 50%;
      }
      .grid-#{$breakpoint-name}-1of3 {
        width: 33.333%;
      }
      .grid-#{$breakpoint-name}-1of4 {
        width: 25%;
      }
      .grid-#{$breakpoint-name}-1of5 {
        width: 20%;
      }
    }
  };
}

All that funky stuff will compile to:

@media (min-width: 30em) {
  .grid-small-1of1 {
    width: 100%;
  }

  .grid-small-1of2 {
    width: 50%;
  }

  .grid-small-1of3 {
    width: 33.333%;
  }

  .grid-small-1of4 {
    width: 25%;
  }

  .grid-small-1of5 {
    width: 20%;
  }
}

@media (min-width: 48em) {
  .grid-medium-1of1 {
    width: 100%;
  }

  .grid-medium-1of2 {
    width: 50%;
  }

  .grid-medium-1of3 {
    width: 33.333%;
  }

  .grid-medium-1of4 {
    width: 25%;
  }

  .grid-medium-1of5 {
    width: 20%;
  }
}

@media (min-width: 60em) {
  .grid-large-1of1 {
    width: 100%;
  }

  .grid-large-1of2 {
    width: 50%;
  }

  .grid-large-1of3 {
    width: 33.333%;
  }

  .grid-large-1of4 {
    width: 25%;
  }

  .grid-large-1of5 {
    width: 20%;
  }
}