This is what I used before I bought dedicated gear to handle my VPN access and allowed me to use the iOS native VPN feature. Basically, you setup a VPN server on your Pi and open the port for the VPN. Then you enter the VPN credentials on the client and you are good to go!

Below is a step by step guide for Ubuntu and tested on Debian. Pi is based off Debian so this should work fairly well although I haven’t tested on the specific Raspberry Pi distribution.

Guide: http://ubuntuforums.org/showthread.php?t=1645473