Make yourself a very thin, very cheap iphone wallet. Everything else out there sucks and/or costs money. Could also work for android phones, or just about anything that is flat.
Credit cards, ID, drivers license, keycard - whatever - under control. This thing will hold 1 card as well as it holds 5. You can stuff in some cash too. You can probably stuff in a key. Or a guitar pick, or a toothpick, or a q-tip or a golf pencil... you get the idea.
Step 1: The Stuff You Need
1. old bike tube.
2 inch tubes work the best, like from a mountain bike or a kid bike. Mine is from a fancy stroller - so it is probably better than yours. Any bike shop will almost certainly give you one if you don't have one.
2. thin card.
Use any thin credit card-sized card. Paper ones won't work, but anything laminated or plastic should be fine. AAA cards, library cards, gift cards, etc. Find one as thin as possible to avoid extra bulk. Mine is a NYC metro card so that everyone knows that i used to live in the city and I am still that cool. I think it even still has money on it - I don't even care.
3. double sided tape
The stickier the better.
Step 2: Cut the Tube
Cut a 1 inch wide piece of tube and slide it around the card. The cleaner the cut the better - frayed edges will make the whole thing rip sooner than you would like.
Step 3: Cut and Stick the Tape
Cut two pieces of tape and stick them to the back of the card on either side of the tube. It doesn't habe to be an exact fit, but try to get it as close to the edges as possible.
Step 4: Remove the Tape Backing
I hate this part, trying to get your fingernail under the edge of the tape while not peeling it up from the card - it is definitely the low point in this whole process. I apologize for making you do it.
Step 5: Stick It!
Here is the grand finale - put it on the back of your phone. I bet you didn't see that coming. Center it, don't cover the camera...it is not rocket science. Give it a good rub to make sure it adheres well.
Step 6: Done
You are done. Put in a few cards and be amazed. I have 4 cards in here, but like I said cash and other stuff fits as well. The fuller you stuff it the sooner it will rip.
Yes, that's right it will eventually rip and you'll have to make another. Mine typically last about 6 months with 4 cards. But that is less a testamant of the product and more of my own awesomeness.
PS - I've been using it for a year and no problems with credit card magnet stuff.