on a black and white canvas, take the pen tool and draw what kind of shape you'd like to achieve, fill in the shape you make, then go to Edit > Define Brush Preset, name it and press ok.
jezek - thx bro, i do like the extra color additions!
codman - lol. o my bad hehe, most of my brushes that i make i just make a circle with the pen tool, i make a few copies, stretch/skew them, then merge them together.
in the above sig, i used the leaf brush (comes w/ ps), i motion blurred it, then i used one of my brushes as the eraser tool and start taking swirls out of it, (my gosh, i'm giving away my tips )