I started making concrete leaves by following instructions I found on the internet, but the best tricks I learned were from Portland artist Roberta Palmer. Her leaves were for sale at the Hardy Plant Society of Oregon sale, held in April http://www.hardyplantsociety.org/ each year at the Portland Expo Center. She had a signup sheet for workshops at her booth: she does leaf casting and leaf painting workshops. I learned many important things, but the most critical one was: STAY AWAY FROM THE EDGES OF THE LEAF; she stops the concrete a 1/2″ or more from the edge of the leaf, so the concrete mix doesn’t ooze over the edge and make a big mess. You get a nice clean edge that you don’t have to mess with; once the concrete leaks, it’s almost impossible to clean up.
I also took one of her leaf painting classes, but have never approached the subtle, magical look of her leaves. She’s a master of dry brush painting and blending colors, and her painted Gunnera leaves are stunning. For painting my leaves (see above) I use Rustoleum Exterior spray paint for the base coat, then Behr Exterior Paints for subsequent coats. You can get 8 oz. sample jars of Behr Exterior paint in custom colors for $4 from Home Depot or Lowes.