How do I keep leftover tomato sauce from spoiling? I made pizza today.  The last time I did this I had some leftover tomato sauce; when I opened that it went POP and smoke came out of the jar.  Tasted really bad, too, so I threw it away and opened a new jar.

I keep having to do this.  I close the jar tight and put it directly in the refrigerator, but it still goes bad.

What can I do to prevent this?  Is there some sort of preservative I could add to the remaining sauce to keep it from spoiling?
Put leftover sauce in a zip-style freezer bag.  Seal most of the way.  Lay on counter and gently remove air, sealing the rest of the way at the last moment.  Freeze flat.  Freezing will extend the shelf life.  Freezing flat will allow for easier storage, and faster thawing.