Let me start by saying that we don't usually recommend shooting with a smartphone. If it's something that your clients and customers will see, we recommend you hire a production company like us to create your videos (that discussion will appear in a different blog post). But on occasion, you may be the only person in a position to get the footage you need, and the only video camera you have is your phone.
One of our clients was in that position recently (they had people in Central America, and there was no budget to send us down to capture the b-roll or interviews for that part of the video). They also had no access to additional equipment (such as lights and microphones), making the situation even more problematic. They asked us to provide some tips to help make the footage as usable as possible, so we created a short primer on the subject.
And since you might find yourself in that position, we've provided a PDF file of the primer here. "How to shoot video (and photos for video) with your smartphone - with no additional accessories".
Let us know if you find it helpful.