WordPress pretty permalinks /w Pretty query parameters (For sort) via ModRewrite

I currently have wordpress pages that that can be viewed via both the index page (all the posts) and also the category pages.

My current permalink structure is ‘http://www.mysite.com/%postname%/‘ which is neat and i would like to keep.

Current examples..

Home page (post list)
http://www.mysite.com/

Category page (filtered post list – filter on category)
http://www.mysite.com/categoryname/

Any post list needs to be order (by choice of the user) by any of the following parameters…

Oldest->Newest , Newest->Oldest , Expensive->Cheapest , Cheapest->Expensive , HighestRated->LowestRated , LowestRated->HighestRated

I have handled all the sorting correctly in the functions.php and it retrieves from from GET parameter set. (the last 4 are custom meta attributes which I have sorted/handled correctly)

Examples are…
http://www.mysite.com?SortBy=Newest

http://www.mysite.com/categoryname?SortBy=HighestRated

I can’t get a correct functioning .htaccess file with ModRewrite that achieves this….

I would like them to look like these (for the 2 examples above)

http://www.mysite.com/SortBy/Newest/

http://www.mysite.com/categoryname/SortBy/HighestRated

My webserver is running Apache on Linux server.

My current .htaccess file looks like this…

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

It doesn’t seem to be doing my current re-write actions but I presume these are handled by WordPress elsewhere??