How can I ensure that reaction volume is not lost due to evaporation during thermal cycling?

Be sure to carefully and completely seal the qPCR assay plate with fresh, optical, thin-wall, 8-cap strips or adhesive optical film before the plate is placed into the real-time cycler. In addition, refer to your instrument's user's manual to determine whether the real-time cycler manufacturer recommends use of a plate compression pad during the run.