Ordering Results

what is an easy way to go about sorting my search results through links, you know, like clicking the link at the top of a column. i know how to pass the order by in a querystring:

http://www.example.com?lastname=smith&firstname=john&state=GA&orderlast=desc

where orderlast is the order querystring.

but what about all the other links? what do I do with the previous querystring entries like lastname, firstname, and state? i find if i add orderlast to my lastname column and i click on it, i will have 2 orderlast querystrings