random_page_cost Parameter
This parameter is common to tune, but explaining what it does requires a lot of background about how queries are planned. Particularly in earlier PostgreSQL versions, lowering this value from its default—for example, a reduction from 4.0 to 2.0—was a common technique. It was used for making it more likely that the planner would use indexed queries instead of the alternative of a sequential scan. With the smarter planner in current versions, this is certainly not where you want to start tuning at. You should prefer getting better statistics and setting the memory parameters as primary ways to influence the query planner.
constraint_exclusion Parameter
If you are using PostgreSQL 8.3 or earlier versions, and you are using the database's table inheritance feature to partition your data, you'll need to turn this parameter on.
Starting in 8.4, constraint_exclusion defaults to a new smarter setting named partition that will do the right thing in most situations without it ever needing to be adjusted.