RNAlater is one of the recommended reagents for the purpose, or else you can also send your RNA samples in ethanol precipitated form but that causes reduction in yield of RNA after purification

I agree with the above answers, RNAlater (Invitrogen) is the preferred reagent for preserving rna for long duration.

Here in our laboratory in Brazil we use RNAholder (BioAgency, São Paulo). Samples can be stored at 37°C for one day, at 25°C for one week, at 4°C for one month, or at –20°C or - 50°C indefinitely. But you can use RNAlater (Invitrogen) that preserves for the same time.