Wear a red string if you wish, but not as a magical talisman.
Nearly everyone has seen the red string. If not on Madonna's wrist, then on Demi Moore or Ashton Kutcher's. Or on half the people walking around Manhattan.
What does it mean?
One of the items required for building the Mishkan, the portable Tabernacle that accompanied the Jewish people in the desert, was red thread (Exodus 26:1). The red dye came from a type of worm, teaching that even the lowly worm has a role in God's dwelling place. From here, the red thread worn on the wrist was meant to remind us to gain inspiration from God and keep our egos in check.
Unfortunately, as with so many things, the meaning of the red thread has been corrupted. Instead of a reminder to bring us back down to earth when get a bit full of ourselves, it has turned into a charm, a protective shield. A little red thread can supposedly protect us from sickness, theft, accident. It can find us a job, a spouse, a baby.
So all you need to do is take a lot of red thread, wrap it around Rachel's Tomb a few times, recite some blessings, and voila -- a magical talisman. Which is a blatant violation of Torah, and miles away from its original intent.
On Your Doorposts
Similarly, the mezuzah has also become imbued with special powers, according to some people. The commandment to place the words of God "upon your doorpost," is a literal reminder to us. "Hear O Israel, the Lord is our God, the Lord is One" is our daily affirmation of belief in one God, our statement of faith that God rules the world, and our commitment to follow the laws that God gave us.
Yet somehow, the mezuzah has been reduced to a good luck charm, protecting us from evil spirits. Think mezuzah as rabbit's foot (which has never been very lucky for the rabbit). Some wear a mezuzah as a pendant on a necklace, and others place a mezuzah on the dashboard of their car. While this may be a nice display of Jewish identity, it is not at all related to the specific injunction to "put these words on the doorpost of your house."
In Guide for the Perplexed, Maimonides specifically includes the use of amulets and charms in his definition of idolatry. Imbuing these items with these special "powers" dilutes the true source of any power, which is God. There are plenty of people with red strings who have problems. And yes, there are many stories of people who have an injury or illness, put on a red string, and find that their injury heals. Of course, it's not a coincidence, but neither should we believe that the red string magically healed them.
All these symbols are reminders that everything comes from God.
Jews don't knock on wood, and don't throw salt over our shoulder. Okay, we should not walk under a ladder -- but that's a safety issue! So feel free to let a black cat walk across your path. Wear a red string if you wish, to remind you of our matriarch Rachel and her holiness, or to remind you to be humble.
Make sure the mezuzah on your doorframe is kosher, because if it's not, you're not fulfilling the mitzvah of "place these words on your doorpost." Touch your mezuzah when you pass it as a reminder that the words of God are a guide for how we should live our lives. Wear your chamsa as a reminder to not be boastful, so you shouldn't cause others to be jealous. Use it as a reminder to not give others the "evil eye," to not be resentful of the success of others, to not covet what others have.
That's what all these symbols are -- reminders that everything comes from God. Any protection, any success, any blessings, even any failures -- it all comes from One Source. With these symbols, it's what they invoke within is that is the greatest power of all.