Easy method of forcing a browser cache update

For years now, I’ve dealt with the problem of browsers caching external documents (stylesheets and their like) by appending the query symbol (the question mark [?]) to my URLs in the source code. As it’s now a different URL entirely to the browser, the browser will download it again and store the particular resource as a new entry in its cache. When I need to update the files again, I usually append and increment a number after the ?, like so: ?21, where the next would be ?22. Sometimes I feel creative and add a random string after the question mark, rather a boring number. It works great, and it guarantees that your users won’t see an old stylesheet or old JavaScript file with an updated web page.