The wonderful Kirsten McKee, a physician and photographer in the UK who happens to be married to my friend Tom (and thus I get to also enjoy her photos on Facebook), wrote this straightforward and highly useful guide to taking reliably decent photos on a smartphone, such as the iPhone.
Kirsten shoots film (instant and developed, small and large), DSLR, and smartphone, so when she offers advice, it's across the breadth of shooting, not just one aspect or technology.
I wrote an essay called "Focus, Damn It!" aimed at freelance writers, trying to explain why a smartphone isn't typically a good choice for documentary or profile photography, because it's difficult to get the right circumstances in which an iPhone, Android, or other device truly shines. You can take great photos with a smartphone, but not all the time. The consistency is the problem.
Kirsten provides the directly complementary view about how to make the right choices and set up shots whenever possible to achieve the best possible results — and how to get inspiration, too. We are yin and yang on this and I agree with all of her advice.
There is nothing as distracting as a wonky picture, particularly when you’re looking at pictures arranged in grids or streams on rectangular slabs of glass. If you shoot the image straight in the first place, you won’t have to straighten it later. And the discipline of checking the horizon lines will also make you more aware of composition and symmetry.