Jim from Shelburne, Nova Scotia sent in this tip. After cleaning your paint brushes, and admitting that you never get them perfectly clean, when you hang them up with the bristles hanging down, a bit of the stiff stuff flows down to the tips of the bristles. The first photo shows two brushes cleaned in the same way and then hung differently to dry. You can see that one is soft and the other stiff.
If you hang them up-side-down to dry, that stuff will stay stuck at the base, leaving the tip of the brush as clean and soft as it was while you were cleaning it.