The 2 basic ways are:

1. Change the prop. APC props are quieter than MAS props. The pitch AND shape of the prop have a big influence on noise. Also a stiffer prop will be quieter than a more flexable one. (Compare the difference in flexability between an MAS and APC prop )

2. The muffler. The stock muffler is USUALLY the most quiet, however, if you have access to other mufflers try them. Occasionally you will find a different (or aftermarket ) muffler is quieter than the stock one. (JTec upright/inverted muffler is quieter than a stock muffler on an OS 1.60 FX by about 3 db )