How can I reliably activate vocal Star Power in Rock Band?
You don't need to shout into it - just any loud noise when there isn't meant to be singing. Maybe hold it near the drummer :P
In all seriousness, I usually just hit the mic too.
I have two techniques, depending on the atmosphere I'm playing in:
Say "Activate!" in a sharp low-pitched monotone (think 50s movie robot). I saw a guy doing this at a Beatles Rock Band competition and while it was hilarious, it always worked, and I've picked up the habit.
If you want to activate right after the end of a vocal phrase, just hold the last note, getting louder as you hold it. This isn't as reliable for me, but it's close, and it doesn't break the mood.
Sing loudly, continuously into the mic through the entirety of the window. The easiest way, I've found, is to just continue singing the previous note into and through the star power phrase; as long as you're loud enough to register, there's a drastically lower chance that the game will "miss" the noise pickup.