There are two ways to approach this:
1- Get an Otter Box- this will protect the phone from falls, etc. but is quite bulky. Even my husband who is no style guy stopped using his OtterBox after a week.
2- Get Apple Care- cost is $100 for two years (the length of a typical iPhone contract) and extends the warranty and includes two "falls"- example: my husband dropped his iPhone 4S down a concrete stairwell and got a replacement for $50 (the new AppleCare replacement is $79).
(I also used AppleCare to replace a bad battery out of warranty free and my daughter's phone would not start up- also replaced free though out of warranty).
If you should drop your phone after two years (when the AppleCare is over) you will likely be eligible for a reasonably priced upgrade.
My daughter got a 5S for Christmas, got a cute case (protects against scratches but that's it), and bought AppleCare.