Tip 3: Go for intense color!
iPhone photos are naturally a little muted and dull. When you layer a filter from most apps on top of that, it will give you an image that's still muted and dull, but now with a color filter over it (usually a yellowish tint). No bueno.
To get intense color without a heavy filtered look, my favorite method is first boosting the brightness and contrast just a little bit and then using a filter from VSCO Cam. It's by far the best app for getting color to pop. The filter I use most often is C1 (which comes with the free app), and I usually turn it down between 25% and 50%.