update on a large table

I need to do an UPDATE on a large (100 million record) table. Is there
any way to speed up the process (Like turning off the transaction log)?

So far postgres has been handling the large database exceptionally well
(large \copy imports and WHERE clauses w/ multiple params) but it is
killing me on UPDATES. It takes about 4 hours to run an UPDATE (the WHERE
clause is against an INDEX) but about 50 sec for a similar SELECT.

Thank You,
-Aaron Held