Reaction Mix: Template and Primers
The proportions of the reaction mixture have a huge influence of the quality of PCR results. There is a general formula for concentrations of template, enzyme, primers and nucleotides to use, but this can be tweaked a little bit. Optimal primer concentrations are between 0.1 and 0.6 micromoles/L. The amount of template varies depending on the type of DNA (human, bacterial, plasmid). With very low amounts of template there are other strategies to improve results, like increased cycle numbers or use of "hot start". Always test new primers with a positive control reaction, to be sure they work under your specific experimental conditions.