Rattlesnake rattles in fiddles have been mentioned a few times.  I always found such folklore interesting and part of the fun of being a musician.  Whether one believes in or follows such superstitions doesn't keep it from being fun or interesting sometimes to hear of them.  These are a few I've heard of over the years..

Rattlesnake rattles.  I didn't know that fiddlers or violinists used that one.  But banjo players and one or two guitarists I've known over the years had one in their instrument.  Some said it was just for luck, others said it was to make people not think of stealing or messing with your instrument.  One banjo player I knew years ago, claimed it "Puts a little extra sting into the sound.  You know, to catch the audience's attention.."  Not sure if that was an actual sound quality he meant or something "magic".

Gator paws.  Some say those are for luck and some say they are supposed to protect.  Only ever saw those used by guitar players or singers.  I knew a singer once who carried a gator paw that was clutching an old silver dime that he said was for money luck.

Silver dimes.  Since we just mentioned those, some musicians have a silver dime hidden somewhere in their instrument or case.  I've heard that is a money charm, so if anybody gets paid or tipped at the gig, it will be you.  One musician I asked about it was maybe a bit less optimistic and said it was so you always had a dime to call home if you ever decided to just give up on music.  

2$ bill.  That one is supposed to be so you always have a little extra money for some fun.  Apparently years ago, 2$ was supposed to be the going rate at a house of ill repute, or some said for a bottle of whisky.  So it was "fun money" and a sign to have a night on the town if you got one in your change or pay.  Keeping one hidden in your case or instrument was maybe supposed to draw that sort of luck or something, I guess.

Chicken foot.  That is so you can always find a gig or opportunity even when nobody else could.

Bible.  Well, the only ones I actually ever saw were New Testaments, usually tucked away in the accessory section of a guitar case.  That was supposed to keep the devil away from your instrument.   I was never sure how that was supposed to actually work if it had been swiped from a hotel or motel room, which the ones I saw appeared to be.  I'd think a stolen one would be more likely to attract the devil, but I may just not understand these things.  LOL

Blade of grass or a wildflower.   I've only seen those with flute players, some of them said you pick it when you are playing on a summer day and keep it in the case with a flute and you'll always be able to play a little sweeter.

That's all I can think of offhand for music specifically.  But for performance in general..

"Break a leg".   It's supposed to be bad luck to tell someone "Good luck!" right before they go onstage.  So you say "Break a leg" instead.

Kissed and kicked.  Some performers (mostly actors) want to be kissed and kicked (not too hard on the kick, I'd guess) right before they go onstage.  One older actress I asked about that once explained that then the best and worst things that could happen from the show had already happened and were out of the way, so the show could go on like it was supposed to.

Peacock feathers.  Supposed to be bad luck for perfomers to even see one before a show.

Bad last rehearsal = a good gig.  Some performers I've known considered it a "good sign" if the last rehearsal before a gig went crappy.  And some felt it was a sure sign that the show would bomb if that rehearsal went well. 

 That's all I can think of offhand.  Anybody else know of any?