The easiest is probably lowering the max freq, as higher frequencies require more volts. Many people find that they notice no performance difference in day-to-day use using 200MHz-800MHz, for example.
If you're running a custom kernel, changing the CPU governor to lulzactive or smartassv2 may also result in an improvement as the CPU will spend less time at higher clocks before stepping down.
p.s. if you're running a Siyah kernel I advise using Voltage Control and (optionally) ExTweaks, as these apps have been developed alongside the kernel and allow much finer-grained tuning.