Is there an existing messenger line? If so use it, but make sure you leave it in place for the next time -- there's always a next time.
If there isn't a messenger line, you need to feed one through. If you were replacing an existing wire, you could use it to pull the messenger line through -- again leave it there when you're done. Alternatively, you could pull it through with an existing wire, then pull the old, new, and messenger lines back through all at once.