How to create a regulated 5V power supply from an unregulated 5V power supply
What you want is reffered to as a "non-inverting buck-boost converter". There are a few different topologies of switched mode converter that can achive that.

Unfortunately all of them are more complex than a simple buck converter (only steps down) or boost converter (only steps up). 

A quick google found a Ti appnote comparing the options. http://www.ti.com/lit/an/slyt584/slyt584.pdf

National semiconductor have an online tool called webbench that will design this stuff for you. Only downside i've found with it is it likes to pick parts that are a PITA to solder.