DH came up with this one all on his own! We use big glass gallon jars with the small necks for our milk but this tip will work for any jars you might have. He wanted to use one of the gallon jars but it had mold on the bottom and we had no way of getting that mold out until...he used this solution:
1/2 cup baking soda (he used a funnel to get it in the jar)
1/2 cup white vinegar
Just put the lid on and shake. The mixture fizzes up and does a wonderful job of getting out any yuckies in the bottom of the jar. Then just rinse out and flip the jar upside down to dry.
This worked great and it means we won't be throwing out any more small necked gallon jugs. It also gets off the dried milk that seems to form just under the curved edge of the jug.