Best way to optimise disk performance in OS X

"Defraging (you'll need a third party app such as iDefrag) may make a difference in specific instances (specifically if you're running at more than 90% capacity or use particularly big files):

http://support.apple.com/kb/HT1375

Some apps (such as iDefrag) also offer disk optimisation and this MacFix article suggests that they may be right getting some performance improvements on a machine that didn't meet the apple criteria:

http://www.macfixit.com/article.php?story=20070301091515843

I can't personally vouch for it but I am interested and may look at buying it when I've got some spare cash knocking around."

This is probably a bit beside the question, but the best thing I ever did to boost disk performance on my Mac was to buy an SSD. Expensive but extremely rewarding results.

"I found that the command iopending (run with root priviliges) gives an interesting overview of the average disk queue length (= pending disk IO operations).

The rule of thumb is that it should not exceed 2. If it does, the storage subsystem cannot handle the load sufficiently."