After a bankruptcy, people often ask their Chester County bankruptcy attorney how to improve their credit. One definite way is by "piggybacking". Essentially, someone gives their credit card permission to "piggyback" off of their good credit by making you an authorized user. The person does not have to physically give you a card. You just ride on the hopes that they maintain good credit behavior.
You can still piggyback off of someone's credit. There was a bit of drama about 6 years ago as some "industrious" businesses started facilitating the sale of piggybacking. My biggest warning to everyone -- make absolutely sure the person will keep paying their bills and maintaining their good credit. You absolutely do not want to have a situation where the person hurts you down the road. You must get a good, positive read on their potential.
Also -- be honest with them. Tell them you don't even need to touch the credit card. You just want to get some wind in your sails to get a good jump-start on rebuilding credit.