Here is a great TOPIK study site. Consider his page the holy grail of TOPIK study sites. The author has done a lot of work in creating a very usable study resource and has done well to explain in laymen's terms exactly what the test is and why anyone should care.
click on 시험안내 on the left hand side of the window. From there,
click on 기출문제 on the right hand side of the window (icon is a manila folder)
There you can download previous tests and mp3 listening portions
or you can read Hanguk Drama's much more visually appealing step-by-step guide.
*Update: another great TOPIK blog has surfaced and is worth checking out