Don't Be Where Your Opponents Expect You To Be
Shoot. Move. Shoot. Move. Shoot. Move. You get the pattern?
Don't stay in the same location after you shoot. Besides your current target now having a reticle where your head is or will be when you shoot again, people you are unaware of may be moving in and getting ready to shoot you as well. You also become susceptible to well-timed grenades.
Shoot To Kill
This may sound like a no-brainer, but I'll explain: if you don't think you can kill the target.. don't shoot. This is generally an issue when spotting targets at a long distance, but can also apply to spotting someone moving quickly around cover.
In duo or squad games, long fights with combat tactics and maneuvers are a thing and wearing an opponent down is possible. In solo, no one has your back and shooting a guy to what amounts to a Jar of Food worth of damage doesn't accomplish much and reveals your position to everyone around you (while your target is now probably going to be ignored).
If you see an opponent, and they don't see you, use that opportunity to gets closer or wait for them to stop moving to get that kill shot.