Best practice for testing new code

I am bad, and I generally make code edits to my theme live, which at times, when I am mucking around in the php, can cause runtime errors.

In most cases, I am working on the index.php file in themes, or search.php, you get the idea.

I have tried to make a copy such as index2.php in themes, and load that, no love. Is there a doc or tech note, on when dealing with a live blog, and changed not warranting setting up a totally new dev system, how do you manage a singe copy of a page to make code changes and not affect regular traffic?