Step 1: Get a USB stick (for older Linux distributions 1GB is enough, newer ones require more than that)
Step 2: Get a Linux distribution (whatever you like. I used Linux Mint 17 Qiana)
Step 3: Make that USB bootable ( http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx )
3.1 Erase the USB stick
3.2 Convert the Linux .iso file to .img by running the following command in terminal app:
hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso
OSX might suffix the .img file with .dmg. Just remove it from the file name.
3.3 List you devices and determine which one is the USB stick (e.g: /dev/disk2):
3.4 Unmount the USB stick:
diskutil unmountDisk /dev/diskN
Note that N is you device number (e.g: 2)
3.5 Write the image to the USB stick
sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m
Again, N is your device number. Prefixing the 'diskN' with r and adding 'bs=1m' will increase the write speed.
* 3.6 Sometimes OSX will complain that it does not understand the filesystem and ask you to eject the drive, the other times you will have to eject it:
diskutil eject /dev/diskN
4. Reboot the system holding the Option (Alt) key during POST(start-up). You will be prompted with a menu from which you will be able to select the boot drive. Chose the USB stick ('EFI Boot'). Now Linux should fire up.
5. Enjoy it.
This was executed on a MacBook Pro 13" 2012 running Mavericks 10.9.5.