Slightly sanding the rubber so that the ink has "teeth" to hold on to helps...should help a bit. Not so much that it "gouges" the rubber.
Also I find with larger stamps (like background stamps) or stamps with large solid areas that it helps to have a good surface under the paper that you are stamping on - a softer surface like a foam sheet or a magazine (rather than your solid table) helps to apply pressure over the entire picture.
This is kinda what I mean by a foam sheet - not too thick or thin but it will have more "give" than your hard surface table: Creatology
-- it allows you to apply a little more pressure over the whole stamp instead of only one area.