Lately, creating brush sets in .abr format has been my favorite means to manage my brush collection. A single .abr file can contain multiple brushes, which makes it a lot easier to manage large amounts of them. Gimp 2.4 and higher can also read brushes from .abr files, so there are no longer any compatibility issues in maintaining brushes between Photoshop and Gimp. I feel that the .gbr brush format used by Gimp is dead, so I no longer bother with it. I also hop back and forth between Gimp and Photoshop a lot more then I used to, so it is great to have both pieces of software reference the same set of brushes. My typical method for creating brush collections is to create my brushes as .png files and then convert them to brushes in Adobe Photoshop. Lastly, I use the Preset Manager to turn a large number of brushes into a single .abr file to keep in store in my collection.
To turn your freshly made .png into a brush, a how-to really isn't necessary anymore. Just go to Edit - Define Brush Preset... and then name your brush!
Your new brush will now appear in the brush palette. Now in the Brush Presets, click the rightmost right arrow and select Preset Manager in the pull-down. The following dialog will be displayed to allow you to manage your brush presets:
Here you can select multiple brushes, so select all of the ones you want to save together and click Save Set... Save your .abr file somewhere in your collection. Now if Gimp is set to use these folders for brushes, it will recognize them as well! Can't get much simpler then that!