We have used three different types of string to attach our cards to our balloons: kite string, sliver gift wrap ribbon, and orange flagging tape.
Normal kite string is inexpensive and works fine, but doesn't make your card any more noticeable to the people who you want to find it.
Sliver gift ribbon is reflective and seems more noticeable on bright and sunny days.
Orange flagging tape is normally used for construction work. It is tied on trees to mark off areas. It is probably the most visable of all the strings we have used; both while the balloon is in air and on the ground after landing. A roll of orange flagging tape costs about $5 at Wal-Mart.
If you have any questions or comments about string, please e-mail me.