The present invention provides methods of enhancing the efficacy and specificity of RNAi by priming RISC activity in cells, cell extracts, and organisms using priming agents such as siRNAs as well as other nucleic acids. The invention also provides priming agents, extracts and cells with high levels of primed RISC activity and therefore responsiveness to RNAi, and methods of using the same in research, diagnostic, and therapeutic applications.