Linseed oil
Alternatively, you can use linseed oil, safflower oil, or even olive oil. It is best to use the purest form of oil for best results.
Old Fashioned
The old fashioned way to clean a paintbrush is to place it in a protective container or keeper. You can also wrap the bristles in tissue to keep them in shape.
Plan
If you plan on using the brush again, you might want to spruce it up a bit. This is also a good time to rinse the brush in clean water.