If you are using a dye based ink you can mist the stamp lightly with water after inking - I did that on this card and the whole image stamped very nicely. The key is 'lightly' mist... if you get too much water you'll end up with a muddied mess! Ask me how I know.
Fall quick card by dini - Cards and Paper Crafts at Splitcoaststampers
Another tip I have heard, is to press your stamp down for 10 seconds, so the ink gets a good chance to soak into the paper. Different papers take inks differently, too.