How to get dependent values based on another Lookup selectbox ?

Hi ,

My requirement is 

I have a table called countries which contains country and city names . Two Lookup selectBox I am using 1 for Country and another Lookup selectbox for city .

Once I select Australia in first control in the second Lookup box it should filter and show   Sydney,Perth,Melbourne same for other countries . How to I apply filter using Reference qual or any other way .

Country           City

Australia          Sydney

Australia         Perth

Australia         Melbourne

America          Austin

America          Seattle

America          Boston

Newzealand    Auckland

Newzealand    Wellington