Easy methods to get the parameters on web page and keep away from 404?

I’ve a web page like http://instance.com/mypage

and I wish to set some $_GET parameter like that:

http://instance.com/mypage/firstparameter/firstvalue

I would like firstparameter worth however I at all times finish within the 404 web page.

I’ve spent some hours with get_query_var(), add_query_vars_filter() and add_rewrite_rule() however no means…

Thanks