What is the correct way to start a mongod service on linux / OS X?

Edit: you should now use brew services start mongodb, as in Gergo's answer...

When you install/upgrade mongodb, brew will tell you what to do:

To have launchd start mongodb at login:

    ln -sfv /usr/local/opt/mongodb/*.plist ~/Library/LaunchAgents

Then to load mongodb now:

    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mongodb.plist

Or, if you don't want/need launchctl, you can just run:

    mongod

It works perfectly.

With recent builds of mongodb community edition, this is straightforward.

When you install via brew, it tells you what exactly to do. There is no need to create a new launch control file.

$ brew install mongodb
==> Downloading https://homebrew.bintray.com/bottles/mongodb-3.0.6.yosemite.bottle.tar.gz ### 100.0%
==> Pouring mongodb-3.0.6.yosemite.bottle.tar.gz
==> Caveats
To have launchd start mongodb at login:
  ln -sfv /usr/local/opt/mongodb/*.plist ~/Library/LaunchAgents
Then to load mongodb now:
  launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mongodb.plist
Or, if you don't want/need launchctl, you can just run:
  mongod --config /usr/local/etc/mongod.conf
==> Summary
🍺  /usr/local/Cellar/mongodb/3.0.6: 17 files, 159M