About a year ago I switched over to Bank of America. Since then, when I go out to lunch or dinner with friends I'm always regretting that I have such a boring credit card. In a large group there is always multiple people with the same card. Where is the individuality? As a creative person this really bothered me. So I decided to take matters into my own hands.
Option 1) Spray paint the card a solid color. This works well. I spray painted it solid silver for a few days, then solid black. The black card was well received but I found out that many merchants need to know what kind of card it is. So I looked into other options. Note: you need to remove the spray paint with sand paper, but be careful around the numbers before you try the other options. Otherwise the card starts to get too thick with paint. For my wife's I tried solid pink and stuck a visa sticker on it from option 3 below.
Option 2) Iron on transfer paper: This was what another site proposed. this did not work well for me. The theory is design your card in photoshop and print it onto Iron-On T-shirt Transfer Paper ($10 at your local hobby store). Cut out the design, use spray on poster board glue to secure the transfer to the card and iron on low heat. I did not like this. In all my attempts the card ended up too thick to use at swipe ATMs. Plus, the iron removed raised numbers from the back. Note, the numbers are not part of the card design - I added fake numbers to the picture just to illustrate - you want the raised original numbers to come through which is why this option sucks.
Option 3) Recommended. Buy some quality Inkjet sticker paper. Print your card design and stick it to the card. You can do front and back just on the back, do not cover the magnetic strip. The stripe WILL work through the sticker but having a stick on front and back makes it too thick (or so my wife says). Once the sticker is in place take a knife and lightly scratch back and forth across the numbers to make them come through and continue to press the sticker down around the numbers as they come through. Mine looks and works perfectly Again, the numbers are not part of the card design - I added fake numbers to the picture just to illustrate - you want the raised original numbers to come through. Here is the back: 
I am still experimenting to try and give the card a good plastic look. At first it looked and felt a little to "papery" so I sprayed a coating of gloss on it.