This was one of the problems I went into the local diaper place to solve.. they have this Rockin' Green shaker that's supposed to keep the stink down, but the girl there told me what she did, and it works reasonably well:

mix baking soda and water until it's a thick paste. Add several drops of tea tree oil. Spoon into muffin tins (I use candy moulds because I didn't already have muffin tins, and the candy moulds are cheaper at the bulk barn than muffin tins.. on the down side, they're smaller, so I have to use more of them "per bag" -- I'm using a fuzzibunz zipper bottom diaper pail bag). Wait for them to dry, then pop them out of the tray. Use as necessary.

Baking soda's hella cheap.