PostgreSQL prefers regular maintenance, so please read the recipe that discusses maintenance planning (the last one).
We recognize that you're here for a reason and are looking for a quick solution to your needs. You're probably thinking, fix me first, and I'll plan later. So off we go!
PostgreSQL provides a utility command named VACUUM, which is a jokey name for a garbage collector that sweeps up all the bad things and fixes them — or at least, most of them. That's the single most important thing you need to remember to do — I say single because closely connected to that is the ANALYZE command, which collects optimizer statistics. It's possible to run VACUUM and ANALYZE as a single joint command, VACUUM ANALYZE, and those actions are automatically executed for you when appropriate by autovacuum, a special background process...