Sorry for bumping this old thread but it has quite a high ranking in the search for the issue of using USB with an external power source. I wanted to document the approach I frequently use in case it is useful for others.

First I cut to seperate VIN and USB as shown by Russ:

I then add a connection to the VUSB via, if it's for a breadboard project I'll add a pin like so:

(If it's for something more permanent, I'll attach my connection directly to the via or neighboring pad)

Then if I want to power it via USB I can just jump from VUSB to VIN:

If I want to power the teensy from an external source, I can just connect its VIN and ground pins to the appropriate source: