Design a power supply from USB
The way to do this for a regular USB is to only use the +5V line and ground, do nothing (leave them floating) with the data lines, they are connected to a digital line driver on the other end. 

If you need something other than +5V use a regulator to lower the voltage:  



Source: http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/02_ps3.htm