AmpliTaq is recommended for nuclear genes or organellar templates that amplify with difficulty. A lucky few have produced homemade polymerase by getting ahold of some Taq plasmid, and subsequently producing and purifying their own enzyme. In general, adjust water to make up a total 100 ul reaction. This is variable. Try anywhere from 8ul to 12ul.
Different genes seem to work better with different amounts.
When you take a new tube of Taq out of the Promega box spin it down for a couple seconds to get the solution off the sides. Keep the Taq on ice at all times - it is fairly stable, but quite expensive. Keep your primers cold. Using 3ul in regular double-stranded PCR sometimes increases product. Vortex this well, before adding Taq.