Good advice above: the main thing is to get the BIOS set to make the USB drive the first priority for booting. BUT be aware that certain brands of thumb drive - e.g. Kingston DataTraveler - will NOT boot an operating system even if the computer's BIOS is set correctly as the priority. Kingston mentions this on its website, but it completely confused me until I found it. I reformatted etc. etc. etc. to try to make it work and it just won't. By comparison other drives - e.g. SanDisk - work fine.
TRUST BUT VERIFY any advice from anybody, including me. Mint 17.2 Cinnamon on a Dell laptop. Dual booting Mint 17.1 XFCE / W7 on an Acer netbook. Mint 17.1 Cinnamon via Persistent Live USB on a Win7 Lenovo desktop with peripherals.