Successful RT-PCR requires a high quality, intact RNA template. Use the following guidelines to help prepare this template:
Take steps to eliminate all potential sources of RNase contamination from glassware, plasticware, reagents, etc.
Use purified mRNA as template, rather than total RNA. Starting with poly(A)+ mRNA will greatly increase the likelihood of successful amplification of rare mRNAs, since the proportion of mRNA in a total RNA preparation is quite low (typically, 15% of total RNA from a mammalian cell).