Aluminum Sulphate is the fastest, Sulfur works well. Pine straw, oak
leaves will maintain a low pH , but take a lot of time if you have a
high pH. If your pH is 6 or below, these mulches may be all you need.
If you have a pH approaching 7, most likely there are alot of buffers
in the soil whic will keep the pH high. If which case only shock
treatments with acidifiers will work. Most local nurseries will sell
something like Green Light Soil acidfier. It is in liquid form, mush
easier to get apply accurately than powders.