Until recently, I always checked the tuning of my guitar at the start of each daily practice and found it to be off. I thought it just didn’t stay in tune. I realized recently that I was wrong: it actually stays in tune quite well!
Here’s a tip I haven’t seen in any guitar-related book or website: check your guitar’s tuning after it has had time to adjust to your body temperature.
I accidentally discovered that if I play scales at the start of guitar practice sessions, and then check the tuning about five to ten minutes after the guitar and its strings have had time to warm up, then no adjustment to the tuning is necessary at all: it’s perfectly in tune.
However, if a tuning check is the first thing I do upon picking up the instrument, then it’s almost always out of tune.
So, while many other factors can affect the tuning of your particular instrument, a subtle factor to keep in mind, that I haven’t encountered anywhere else, is that tuning should not be the first thing you do at the start of a practice or play session. Instead, it should be something you do after the instrument has adjusted to your body temperature.