Besides the Dyna-Gro KLN that RosieC recommends, there's Superthrive, used by the eyedropper. Also, mollasses water is supposed to encourage roots, and then there's liquid seaweed. Seaweed can grow as much as a foot a day.
The KLN and Superthrive are powerful products that work but should be used judiciously and according to the label. The seaweed can be used once a month without worry.