I remove rubber rings from the lens and wash them in warm soapy water. They'll stretch enough to pull them off, just try not to stretch them any more than is absolutely required so that they will fit snugly when you put them back on after cleaning. Be sure to let them completely dry before reinstalling them on your lens.

Sometimes an old toothbrush or similar tool helps to get the crud out of the grooves. If the buildup is especially stubborn, I sometimes use lens cleaner fluid as well.

Replacement rings are also usually fairly inexpensive.

I remove rubber rings from the lens and wash them in warm soapy water. They'll stretch enough to pull them off, just try not to stretch them any more than is absolutely required so that they will fit snugly when you put them back on after cleaning. Be sure to let them completely dry before reinstalling them on your lens.

Sometimes an old toothbrush or similar tool helps to get the crud out of the grooves. If the buildup is especially stubborn, I sometimes use lens cleaner fluid as well.

Third party replacement rings are also usually fairly inexpensive.