The best options are non-tradtional overclocking techniques, since Apple has neglected to give us traditional means (ie, jumpers). A list of things to use:
1. Thermal paste
2. Larger heatsinks/fans
3. PCI Slot Fan
(any of the above can be combined)
4. Peltier devices (see arstechnica.com for this one-- air convection currents or something)
5. Water Cooler (water based cooling system, instead of air)
6. Kryoplant (currently very unstable-- uses liquid nitrogen).
(the above three can only be done solo and without the first three).
Doing any of the following will void you warranty, and using a Kryoplant might destroy your computer (if it leaks).