What I did personally was grab a Google image of a muzzle flash and do some work on it to make it a transparent Photoshop image (.PSD file). Now I can just import that file and add blurs/transform it however I need to.
It ends up being pretty versatile...allowing me to do shots like this:
and this as well:
Just a matter of transforming it, and adding the right amount of motion blurs.
Justin