How can I fix my toilet valve so I can stop jiggling the handle after flushing?
You can purchase a new flapper valve if you can find one of the same model, or my preference is to replace the entire guts of the toilet if they are more than 5 or so years old.  Kits and/or parts are easy to find at your local HI store and it's one of the easier plumbing jobs out there (though notoriously dirty).

Just be sure you follow the first rule of toilet repair: do not over tighten or use force.  The porcelain can take a lot of compression, but if anything is pushed to expand, it will crack easily and you're left with buying a new toilet, replacing the wax seal, and possibly the supply line if your existing one isn't flexible.