We often have the tendency to talk slowly and robotic when using voice recognition apps, but you'll get better results if you speak more naturally.
Kevin Purdy over at Fast Company explains:
Digging around Android forums and elsewhere, I found a good number of testimonies from speech-to-text enthusiasts who saw better results from simply speaking at a normal clip. Google representative Nadja Blagojevic offered much the same advice for the search giant's voice product in both Android and its Chrome browser: "Speak naturally and clearly, but don't strain to enunciate too much or speak slowly."
Most of us are so accustomed to typing with a keyboard — and to older, less accurate voice recognition apps — that we think we need to over-enunciate if we want our phone to understand us. It turns out, these apps are designed to understand you the way you'd normally speak. So stop stressing about how you sound and just get the words out; your phone will keep up. Hit the link for more tips on mastering voice recognition.