Installing Postgres+PostGIS has never been easier on Mac. In fact, it is now an app! You download the app-file from postgresapp.com, place it in your Applications folder, and you're done. Really.
If you think that was over too fast
If you think that was over too fast, there is one more thing you can do. Add the postgreapp "bin" directory to PATH.
vi ~/.bash_profile # add line: export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/9.3/bin
Next time you open terminal you will be able to execute all of the following commands:
clusterdb createdb createlang createuser dropdb droplang dropuser ecpg initdb oid2name pg_archivecleanup pg_basebackup pg_config pg_controldata pg_ctl pg_dump pg_dumpall pg_receivexlog pg_resetxlog pg_restore pg_standby pg_test_fsync pg_test_timing pg_upgrade pgbench postgres postmaster psql reindexdb vacuumdb vacuumlo
cs2cs geod invgeod invproj nad2bin proj
gdal_contour gdal_grid gdal_rasterize gdal_translate gdaladdo gdalbuildvrt gdaldem gdalenhance gdalinfo gdallocationinfo gdalmanage gdalserver gdalsrsinfo gdaltindex gdaltransform gdalwarp nearblack ogr2ogr ogrinfo ogrtindex testepsg
pgsql2shp raster2pgsql shp2pgsql
That is pretty f'ing awesome!!