This is my favorite way to cook chicken for recipes that call for cooked shredded or diced chicken breast. It always comes out so moist and tender. This is also one of those recipes you can read once and remember - you don't have to refer back very frequently.
1. Place frozen chicken breasts in pot of water at fill with water just to cover.
2. Add a handful of onions and celery roughly chopped (the inside stocks of celery with the leaves especially). Also season the water with salt and pepper. I just scoop a small handful of salt - maybe 1-2 tsp.
3. Cover with a lid and cook on medium heat until boiling, then reduce heat to low.
4. Leave the lid on the chicken and simmer on low for 5 minutes. Then turn heat off and let the chicken sit 20 minutes with the lid still on.
5. Shred or dice accordingly.
Chicken gets really tough and dry with too much heat so that is one reason this cooks so well.
Let me add a note that 95% of the time I am too lazy to thaw chicken before I cook it and since it turns out so moist and delicious every time, it makes me even lazier. For thawed chicken, you would do everything the same except, just bring it to a boil and turn it off.