psql: Logs me in with my default username
psql -U postgres: Logs me in as the postgres user
Sudo doesn’t seem to be required for me.
I use Postgres.app for my OS X postgres database. It removed the headache of making sure the installation was working and the database server was launched properly. Check it out here: http://postgresapp.com
Edit: Credit to @Erwin Brandstetter for correcting my use of the arguments.