So if you want to keep it simple or are not terribly technical follow these steps to create cascading drop downs within SharePoint 2010.
The first thing you will need is a SharePoint 2010 list to populate the drop downs. Basically it works like this: Each Item and it's respective columns will filter the items in the following drop downs. For this example I have created a 'location' list called 'suburbs'.
There are two other metadata columns that define the suburb: State and Country.
The way the cascading drop downs will work in InfoPath is that the first selection will be 'Country' which will then filter the second drop down, 'State' which in turn once slected, will give the user filtered option in the third drop down; Suburb.
The List
Custom list created with two additional metadata choice columns (Country and State).
List with some sample data:
To create the cascading drop downs in the form you will first need to create 3 empty datafields.
Add the three drop downs of the fields to the form. To populate the form with the columns from the list create a new SharePoint list data connection, which can be found under the 'data' tab.
Now right click on the first drop down and go to Drop Down List Box properties. Select 'Get Choices from an External Datasource'. Select 'Suburb' (this will be automatically selected if it is your only external datasource). For the first 'Country' drop down select 'Country' as the value and display name and also check the box 'Show only entries with unique display names' (this will avoid duplication).
Now right click the 'State' drop down, follow the same procedure as before except this time select 'State' for value and display. However when you select the 'State' option, you will need to click on the 'Select XPath' from next to the 'Entries' box. This will allow you to filter the 'States' based on the selected Country.
Select the 'Title' drop down and then click on 'Select a field or group'.
However, first follow the procedure above to change the third drop down to display 'Suburb' fields and filter them agaisnt the field selected in the 'State' drop down box.
Now to create the rule that resets the other two drop downs when you reselect the first 'Country' one.
Right click on the 'Country' drop down and select 'Rules' > 'Manage Rules'. A new side panel will appear on the right which allows you to manage the rules of this particular field element.