In it's simplest form, the Mixolydian mode is a major scale with a flatted 7th instead of a major 7th. It's use comes into play when you are playing over the 5th of a chord, assuming that that chord is just a Dominant 7th chord. Honestly, the use of Mixolydian (like all modes) best comes into play when you are able to stretch outside of the tonal center. If you are playing over a C chord, and no 7th is present in the chord; than see what happens when you sneak that b7 in there. May work, may not. It all depends on your conviction and the music being played around it.

As far as most modes go, I feel Mixolydian tends to get shoved aside being that it is a scale that roots itself in the Dominant 7th chord. When those chords come up, there are TONS of scales and melodic ideas that you can try that by way of tradition and recorded music work very well.

I would say make sure you have a few scale patterns of Mixolydian under your belt and than try and apply them in your practice routine by looping some chord progressions and seeing what sounds best. Honestly, the rules are only there to teach and from there with the right conviction and confidence you would be amazed at what you could make work. Good luck. :)