Is there a best practice or method to create a sandbox area to test new code of an existing site? The only idea I've come up with is to create a subdomain and copy files from the live site to this subdomain. Then, I could make edits and push back changed files to the live site. It would be nice to be able to push only modified elements back to the live site versus trying to push the entire site back and forth.

Any ideas or thoughts will be appreciated.
Thanks.