Instructables

What is the simplest and most efficent way to convert 3v to 5v?

There are several circuits out there on how to up-vert a 3v input to a 5v input, hwever I would like to know the simplest way.

I would say the simplest thing to do is buy a minty boost kit.
The answer of course depends on how much power you need.

Assuming you only need approximately one watt, or 5 volts at 200 mA, whichever is closer, there exist gizmos sold in the marketplace that can do this for you.

These are usually sold described as a "emergency usb charger", or "emergency cell phone charger".  
http://www.google.com/search?hl=e&tbm=isch&q=emergency+usb+charger

The ones that run on 2 AA batteries, these essentially do what you want, convert circa 3 VDC into regulated 5 VDC, using some kind of switchmode DC-to-DC convertor.