Hi! The ideal concentrations depend on a number of factors--the type of polymerase you use, whether magnesium chloride is already included in buffer and most importantly on the DNA template and the primers used. Usually, the supplier of the different components give some suggestions. For example, I use GoTaq Polymerase and the supplier suggests the following concentrations:

Buffer (without magnesium): 10 ul

MgCl2: 2-8 ul (1-4 mM)

dNTP mix: 1 ul of 0.2mM of each dNTP

Forward primer: X ul: 0.1-1.0 uM

Reverse primer: Y ul: 0.1-1.0 uM

Polymerase: 0.25 ul (1.25 u)

Template DNA: Z ul (0.5 ug/50 ul)

Water to add up to a total of 50 ul reaction volume

But I would start out with whatever concentrations the supplier of the polymerase you choose suggest and then optimize by varying concentrations and cycle conditions (especially annealing temp.) a little. There is no set concentrations/volumes that work for all DNA templates/reaction components.

In my experience the DNA concentration doesn't seem to matter much (but again that depend on locus, organism, etc.). Usually you can go with much smaller volumes than those suggested by the supplier--e.g. I would go with a total of 25 ul (not 50 ul), i.e. half all suggested volumes.

I usually prefer to add MgCl2 separately (and not already included) in the buffer, so that I have more control over it--MgCl2 altering MgCl2 can have a large effect on efficiency of the reaction. 

There are some PCR master mix calculators, e.g. by Sigma that you might find useful too: http://www.sigmaaldrich.com/life-science/molecular-biology/pcr/learning-center/pcr-tools.html

I hope this helps!

Usually the Polymerase u are using, that particular company gives the PCR reaction along with its packaging. Follow that as per instrution, as diff. company might have diff. reactions depending on Polymerase etc.

Thank you guys