On Splitcoaststampers there is a thread about using polyester fiberfill to ink, or "sponge" the edges of cardstock. This is the technique that I have used since reading Carol Heppner's book "Inking". When I first started stamping I used cosmetic wedges, but I ended up with lines. Then I moved on to sponges, but felt that too much ink was soaked up and transferred on to the cardstock and I had little control. Then I read the "Inking" book and now used polyester fiberfill.
One thing that I LOVE about using fiberfill is that it is so cheap.

Here is a tutorial I made about how to ink the edges of your cardstock.
Supplies: Classic ink pad, scratch paper, your stamped cardstock you want inked, and a hunk of fiberfill.

1. Dip your fiberfill into the inkpad...being careful not to push down so hard that your fingernail touches the inkpad (you will see that this happened to me and I have a couple of marks on the cardstock as a result).

2. With your stamped image laying on scratch paper, start on the scratch paper and move in a circular motion toward the cardstock. Keep moving in circles on and off the cardstock all the way around the cardstock.

3. Keep applying ink in this fashion until it is dark enough for your liking.

4. Finish off the card.