If you find yourself wanting to cancel one of you cards, heed this advice first. Before you cancel, see if you can move your credit line to another card that you already have from that same bank. (obviously, if you have another card from that bank)
There are a couple of reasons you should do this.
You can accomplish the above by calling the number on your card. However, do not do it in the same phone call. In the first phone call, move the credit line. In the second phone call, cancel the card. Note, Chase and Amex will reallocate credit. Citi and Cap One do not.
Note, the same logic applies if you receive notification that a bank intends to cancel your card due to inactivity. This has happened to me a few times.