Hi Warner,
The best solution is just to rename the skin and use a version as part of the skin name, like the skin on this site is mojo_v34 and I keep incrementing the _v part whenever I make a change.
Using the latest mojoportal you could even make small css tweaks right from the browser. Under Administration > Advanced Tools > Design Tools, there is a place where you can set a cookie to prevent your browser from caching while you edit (so you don't need to disable caching from web.config), and there is also a way to copy a skin as a new name so after you finish tweaks, you can go into your skin and copy it as yourskin_vx, (where x is the next version) then change the site to that skin and it will make all users see your changes because the skin name changed and therefore the url changed so it is not cached already.
Hope it helps,
Joe