It depends whether there is residual Trizol in your RNA sample. I would often add glycogen and isopropanol to the aqueous phase and incubate it at room temperature for 10 min and then overnight at -80oC to help RNA precipitate. Then, centrifuge as in the protocol, wash pellet with 500 uL of 75% ethanol and centrifuge at maximum speed (21,000 g), 4oC for 20 min, air dry for 1-2 min and resuspend in RNase-free water.