I spent many years in the ambulance service and performed CPR on many patients. I also taught hundreds of basic life support courses and these are some not-so-obvious tips that make a big difference to performance and stamina when delivering CPR.
The closer you can get to your patient, the better your CPR will be. If you need to, move their arm out of the way so that your knees are touching their torso.
Lean forward so your shoulders are over your hands and you are using your full bodyweight to compress your patient's chest.
Chest compressions are very tiring, using your weight efficiently will ensure you can maintain good quality compressions.
Once you are leaning forward, keep your arms straight so that the full force of your weight is transferred to your hands and not lost in the bending of your elbows.
If a patient is unconscious and not breathing normally, call for help and start CPR.