I find ringtones extremely annoying, so my iPhone is typically on vibrate mode all of the time. For this reason, I sometimes miss texts and phone calls when my device isn't in direct contact with me. I hate missing alerts, but I'm still reluctant to put the volume up on my ringer.
An easy, built-in workaround is to enable the LED light on your iPhone to flash whenever you receive notifications. It's the same light used as the camera flash and flashlight, so it's bright enough to see during the day, and perfect for catching alerts. In fact, it's bright enough that I can usually see it flashing through my pockets, and it's a cinch to enable.
Head into your iPhone Settings and tap on General. Next, enter the Accessibility preferences, then scroll down until you see LED Flash for Alerts. Enable this and you're done!
It's an easy preference that's buried in the Settings, but it can make a difference in the way you catch alerts. Also, as a personal tip, either disable this feature or make sure your LED is face down when you're in a movie theater; it can be quite distracting and annoying for audience members around you—trust me.