The additives used in the PCR allow optimizing the conditions of the amplification reaction. There is an extensive list and each must be empirically tested on each system to verify its usefulness.
A description of the additives that reduce secondary structures and other common additives.
Bitesize Bio
A list detailing the mechanisms of action and optimal concentrations of several PCR enhancers.
OpenWetWare
A post about the use, combination, optimization and mechanism of action of different PCR additives..
Royal Botanic Garden Edinburghy