A good start is to grep for "longdb" in your error log and check for queries that are fast under normal conditions.
Another option in general is to switch to other file systems, especially with newer kernels. Here are some options tested with PostgreSQL 9 with some recent Linux kernels, using already data=writeback per default
http://www.phoronix.com/scan.php?page=article&item=linux_2638_large&num=2
One can see, ext3 or ext4 are not among the fastests for TPC-B with PostgreSQL. I would not recommend everybody to switch blindly. Sites with low db activity and traffic won't see any difference.
