max_connections = This option sets the maximum number of database backend to have at any one time. Use this feature to ensure that you do not launch so many backends that you begin swapping to disk and kill the performance of all the children. Depending on your application it may be better to deny the connection entirely rather than degrade the performance of all of the other children.
shared_buffers = Editing this option is the simplest way to improve the performance of your database server. The default is pretty low for most modern hardware. General wisdom says that this should be set to roughly 25% of available RAM on the system. Like most of the options I will outline here you will simply need to try them at different levels (both up and down ) and see how well it works on your particular system. Most people find that setting it larger than a third starts to degrade performance.
effective_cache_size = This value tells PostgreSQL's optimizer how much memory PostgreSQL has available for caching data and helps in determing whether or not it use an index or not. The larger the value increases the likely hood of using an index. This should be set to the amount of memory allocated to shared_buffers plus the amount of OS cache available. Often this is more than 50% of the total system memory.
work_mem = This option is used to control the amount of memory using in sort operations and hash tables. While you may need to increase the amount of memory if you do a ton of sorting in your application, care needs to be taken. This isn't a system wide parameter, but a per operation one. So if a complex query has several sort operations in it it will use multiple work_mem units of memory. Not to mention that multiple backends could be doing this at once. This query can often lead your database server to swap if the value is too large. This option was previously called sort_mem in older versions of PostgreSQL.
max_fsm_pages = This option helps to control the free space map. When something is deleted from a table it isn't removed from the disk immediately, it is simply marked as "free" in the free space map. The space can then be reused for any new INSERTs that you do on the table. If your setup has a high rate of DELETEs and INSERTs it may be necessary increase this value to avoid table bloat.
fsync = This option determines if all your WAL pages are fsync()'ed to disk before a transactions is committed. Having this on is safer, but can reduce write performance. If fsync is not enabled there is the chance of unrecoverable data corruption. Turn this off at your own risk.
commit_delay and commit_siblings = These options are used in concert to help improve performance by writing out multiple transactions that are committing at once. If there are commit_siblings number of backends active at the instant your transaction is committing then the server waiting commit_delay microseconds to try and commit multiple transactions at once.
random_page_cost = random_page_cost controls the way PostgreSQL views non-sequential disk reads. A higher value makes it more likely that a sequential scan will be used over an index scan indicating that your server has very fast disks.
Increase, in postgresql.conf, the value of effective_cache_size.
Run VACUUM on database tables frequently.
Run ANALYZE on the database periodically.
Don't run VACUUM FULL too often.
Move the WAL files to a separate physical disk.
Increase, in postgresql.conf, the value of sort_mem.
Reduce, in postgresql.conf, the value of random_page_cost.
Increase, in postgresql.conf, the value of max_fsm_pages and max_fsm_relations