I would recommend Fingerprint 2 from Collobos Software. From the developers site:
FingerPrint automatically discovers all printers connected to your Mac
or PC and makes them available to your iPad, iPhone, and iPod touch.
For a fraction of the cost of a new printer, FingerPrint breathes new
life into the printers you already own.
The basic premise of the software is that you install it on a PC or MAC and it acts as a simple print server proxy to provide any access to any printers connected to your computer. It offers work flow integration and security (ACL's). You must be on a WiFi connection to be able to print. I use this software daily and it "just works". It's not an inexpensive solution ($19.95) but for the money it is incredibly simple to setup and will have your printing in under 5 minutes.
Developers site: Collobos Software