The invention relates to an apparatus and methods for preventing condensation on the interior surfaces of sample tubes which are being exposed to temperature cycles, such as during a PCR amplification reaction. In particular, the invention relates to apparatus comprising a sample block comprising a plurality of sample wells for receiving sample tubes and heating elements disposed in the sample wells for heating at least a portion of the sides of sample tubes (e.g., at least the portion which forms the head space after a tube is filled with a PCR reaction mixture). In a preferred aspect, the sample block is part of a thermocycling device for performing PCR and the side-wall heater is used to enhance uniformity and speed of amplification reactions. For example, by decreasing or eliminating condensation, signal strength jumps in a real-time PCR assay can be minimized as can reaction non-homogeneity.