1. Choose Your Camera Wisely.
This actually represents two tips. First, if you plan on using your phone as a camera, know that some phones have better cameras than others; take that into consideration in choosing a phone. Second, most smart phones have two cameras: a front-facing camera and a rear-facing one that facilitates video calls. The camera facing front often has better resolution than the one facing back toward you, but the rear-facing camera you makes it easier to see what the camera sees in the display. That makes it easier to see the picture before you snap the shutter.
2. Look Toward the Light.
Again, two tips in one: First, you want to get good lighting if you want good pictures (this rule applies almost universally in photography); second, as front-lighting generally works better, if you face the light source you maximize the likelihood of taking a well-lit picture.
3. Get a Timer and/or Remote Control.
Unfortunately, particularly when taking a selfie, pushing the shutter button on camera phones almost always causes some camera shake, which can take away from your photo's sharpness. You can reduce or eliminate camera shake by using an app that provides a timer for the camera or by using a remote control (usually operated using Bluetooth these days).
4. Your Arms Are Too Short!
Remember when you hit middle age and you found that your arms could not stretch out far enough for you to read comfortably without glasses? Well, unless you have the reach of a Kevin Durant (for those of you unfamiliar with Kevin Durant, his a 7-foot-tall basketball player for the Warriors who has unusual length in his arms) in many cases your arms will not stretch far enough to get you the perspective you want for the picture. You can solve this problem by acquiring one or two accessories. A small table-top tripod used with a timer app or a remote-control shutter can go a long way toward solving the arm length problem as well as the camera shake problem. (Note that you may need a special attachment to hold your camera and mount it to the tripod). A selfie stick can also come in handy, providing a great deal of versatility in framing your shot. A selfie stick comes with a bracket that holds your phone and connects it to the stick. The stick, effectively a monopod that you hold in your hand, operates as an arm extender. Selfie sticks come in several lengths and usually collapse for easy carrying. Many selfie sticks come with Bluetooth remote shutters. Some of the remote shutters come built into the handle of the stick and others as a separate piece. The built-in shutters have the virtue of making it harder to lose the shutter. The separate shutter has the advantage of working with the camera whether you have it set up with the selfie stick, making it a more versatile acquisition. We prefer the separate shutter for that reason.
5. Follow Normal Photo Protocol (and Take Many Pictures).
Experienced photographers will tell you that when you take pictures, you should concern yourself with lighting, composition and framing of the photo. When taking a selfie, you should do the same thing. Experienced photographers will also tell you to take multiple shots. Taking only a single picture decreases the odds that you will like what you get. Taking multiple pictures of the same thing increases the odds you will get one you like. Remember you get digital images and you can see them without having to pay for film processing and printing. You can easily throw out the ones you don't like and save those you do.