I don't think it's really what you say that stabilizes your dream. What vocalizing in dreams does is boost your belief and confidence that you are in fact, dreaming.

You could just as effectively yell out "rat farts!" (IF you have already conditioned yourself to only say that when you were absolutely sure you were dreaming) and your lucid stability would increase just the same.

It's very possible to suspect you're dreaming, say something like "increase lucidity", and not have anytyhing happen due to the fact that you're essencially hoping that the lucidity will increase. That's why (in my opinion) some people find it hard to fly in dreams. They just don't believe enough that it's possible to do, even in their dreams.

Audible commands usually work because most people are already pretty sure or convinced they are dreaming when they use them. Still, it all comes down to what (phrase) works best for you. Really, you just have to believe.