The simplest way for external power is just to use a USB micro-B plug on one end, and your power source on the other end. That way when you are programming, you use the normal computer USB cable, and other times, you hook up the USB cable.
An alternative to cutting the traces that some people use is a special USB cable that does not connect the power, but it provides ground, D-, and D+. Then you can always power the device with 5v and not having to worry about dual power issues when you program it. I've seen products on tindie.com that have a switch so that you can enable/disable the power line on USB.
The specification for power is 3.6 to 5.5 volts. This means you can run it off of a single cell li-po battery (typically 3.7 volts nominal power, 8.4 volts freshly charged). One of the uses of cutting VIN/VUSB is you can insert a li-po charger in between VUSB and VIN. Some designs require you to cut the pads, while others are more passive, and will only provide battery power if there is no power on VUSB. There are several different li-po chargers I've used for Teensys. Let me know if you want a list.
I usually do not cut the VIN/VUSB traces, and instead just rely on only plugging in USB power or external power. Note, if you are using things that consume lots of power (lots of LEDs, motors, servos, etc.), a normal USB connection might not provide enough amps to power your stuff. In the past with other processors, I needed to program the device, and then switch power to the external source to run it.