Keep card rigid. If your entire card is becoming flimsy due to a crack or tear, you will want to reinforce the card as this can cause the magnetic strip to move or slip around when being read by the scanner. This is done by reinforcing the plastic area of the card with some heavy tape, such as duct tape. Be sure to tape the split, but do not cover any of the actual areas of the magnetic strip.
Cover strip with clear polish. If you have reinforced the areas surrounding the magnetic strip, but there is a tear or break directly in this region, you now want to carefully apply clear nail polish or glue to the area. Be sure to cover it in as little amount, and as thinly as possible and let your card then thoroughly dry out.
Put card in plastic bag. After your card has been able to dry out completely, you can enhance the scanning device's ability to successfully read your credit card. Put the credit card inside of the sheer plastic shopping bag and pull it tightly around the card. Remove any wrinkles from the bag and then slide your card through the scanner, and it should work.
Create a new card online. If none of this has worked, you can go online to create a brand new credit card with a replaced magnetic strip. You can find many such sites here: http://www.filebuzz.com/findsoftware/Magnetic_Stripe_Writer/1.html where you will then in turn download the software. Follow the directions that will lead you through inputting the information and then filling out the proper data to build the card. You will then be able to order a new credit card tailored to you and then have it shipped.
Contact bank or credit card company. If nothing has worked, and you were unable to make your own replacement, you can then call your bank or credit card provider to reissue a new card. They will then make your last one void and you can expect a new one to arrive to you in the mail.