Same place
Pick one spot and never change it. A plain wall, a doorway, the same mirror. Mark the floor where your feet go, or use a tile edge or the rug as the line. The background is the ruler every later photo is measured against.
Same light
Light changes a body or a face more than most real change does. A window shifts through the day and across the seasons; a lamp you switch on is the same every time. If you must use a window, shoot at the same hour and close the curtains everywhere else.
Never light from directly above. It hollows the eyes and exaggerates whatever it hits.
Same distance and height
Rest the phone at the same height, on a shelf or a tripod, and stand on your mark. Moving half a metre closer changes proportions enough to fake weight gain or loss. Use the rear camera, and never zoom.
Same pose and clothes
Pick a pose you can repeat without thinking: feet together, arms relaxed, eyes on the lens. Same clothes, or as close as you have. For skin and hair, same head angle, same expression, no smile.
Use an overlay
Doing all of this from memory is the hard part. Journey Tracker puts a faded copy of your last photo over the camera view, so you line up the wall, your shoulders, and your feet before you press the shutter. The guessing goes away, and so does most of the drift.
Same schedule
Weekly for most goals, monthly for slow ones. Shoot at the same point in your day, first thing in the morning before eating, so water weight and puffiness stay out of the picture. A reminder on the same day each week does the remembering for you.
Compare, then decide
Judge photos weeks apart, never days. A slider that wipes between two shows the outline moving; side by side shows the whole picture; a timeline played in order shows the direction. Trust the trend, not the mood you're in today.