I've created cascading drop downs populated from SharePoint lists, by following this post by Lars in SharePoint designer without using code, it works fine, my issue is that the secondary drop down only populates if the main drop down causes auto-post-back, is there any way to make the secondary drop down populate without causing post back and without using server-side code?
I don't get it. Cascading means exactly this: one depends on another. In your case, the secondary dropdown can only be populated once a selection on the first dropdown has been made and sent back to the server with autopostback=true. The query of the secondary dropdown datasource needs to receive the value of the parameter from the first dropdown selection. How could the secondary dropdown populate without it?
The only thing that you could try is to set the DefaultValue property for the datasource ControlParameter.