The suggestion to use `Antialiasing->False` doesn't really solve the problem. I don't have a single solution that's appropriate in all cases, but I think one of the approaches I list on the following web page will work: 
[Avoiding artifacts in shaded contour and density plots][1]

**Edit:**

The following method from the linked article solves the problem:

Instead of exporting the image (assumed to be stored in `im1`), export the modified graphics

    im1 /. {EdgeForm[], r_?(MemberQ[{RGBColor, Hue, CMYKColor, GrayLevel}, Head[#]] &), i___} :> {EdgeForm[r], r, i}

This replaces the invisible edges of the polygons (`EdgeForm[]`) in your graphic (called `im1` here) by edges of default thickness and with a color matching at least one of the neighboring polygons. The new edges then help fill any empty space between the shaded areas.

**Edit 2**

My solution relies on finding colored polygons without colored edges by looking for `EdgeForm[]` followed by a color in the graphic `im1`. 

Based on the answer by Mr. Wizard and kguler [here][2], one can also write make the above work better with custom colors and future additions to the built-in color choices:

    colorQ = FreeQ[Quiet@Darker@#, Darker] &;
    im1 /. {EdgeForm[], r_?colorQ, i___} :> {EdgeForm[r], r, i}


  [1]: http://pages.uoregon.edu/noeckel/MathematicaGraphics.html#ExportShading
  [2]: http://mathematica.stackexchange.com/a/15434/245