I'd love to say I'm one of those photographers who always has their camera on them, but... Well, I'm just not. So I rely on my phone's camera quite a bit, as I'm sure do most people.
Here's some tips I apply when using my phone for photos!
Clean your lens before you capture your image. Crystal clear images come with clean lenses!
When possible, don't zoom - move closer or crop the image. Digital zooms will reduce the quality of your image, as it reduces the resolution. Try moving closer to your subject, instead.
Keep your phone steady. Consider buying a mobile tripod! This is especially helpful in low-light environments.
Use the rule of thirds. I know, I know. Literally every list on this topic has this one. But seriously, it does help! Most phones come with a setting to help with this, dividing the image in to 9 equal parts on a 3x3 grid. (Snapchat even does it now, too.)
If it's time to get a new phone and phone photography is something you're interested in, consider your new phone's camera ability. I recently upgraded and my new phone shoots raw files, which honestly blew my mind a little.