Chiggers are the larvae of a mite that burrow into your skin and release a tiny bit of digestive enzyme that eats some of the surrounding tissue--which is what causes the itch. Once you see and feel the big red lump, the larva is no longer in there. It may have moved to another location on your skin to feast again, which is why you almost never have just one chigger bite.
Apply alcohol to the affected areas. This will break down the remaining digestive enzyme and the alcohol sting should stop the itch. Rubbing alcohol and mouthwash should work equally well.
Rub on some vinegar. Like alcohol, the vinegar dissolves the leftover chigger feeding substance and stops the itch. Reapply as needed.
Buy some Chiggerex. Chiggerex is a commercial product designed to alleviate the aftereffects of a chigger in your skin and is likely available at your pharmacy. Many people report that this is the best way to stop the itch.
Get rid of the larva. By the time you notice the itch, the chigger has likely moved on to another spot of skin. Make sure the source of the itch is gone by using Nix or other lice removal product as your "soap" in the shower, then wash again with your regular soap.
Soothe the itch and promote healing. Strong diaper rash cream like A&D Ointment or Boudreaux's Butt Paste have been used to relieve chigger bites as well as diaper rash. Apply such products as needed every few hours by rubbing into the affected areas.