1. Plan ahead to allow time to bring fish to room temperature before cooking, thick fillets in particular. They'll cook more evenly if they're the same temperature throughout.
2. As a rule of thumb, allow 8 minutes of TOTAL cooking time for each inch of thickness of your piece of fish.
3. Generally, fish should be cooked to an internal temperature of 120F to 135F; non-oily fish at the lower range, oily fish at the higher range.
4. If you prefer rare salmon or tuna, make sure the fish is impeccably fresh, and cook to 115F.
5. Fish is done when a knife is easily inserted into the thickest portion of the fish; leave the knife in the fish for a few seconds then quickly touch the tip of the knife to the inside of your wrist. Cold- not done; warm- perfect; hot- it's overcooked and you need to get it out of the pan and on a cool platter at once.
6. To prevent fish drying out when broiling, first coat it with a thin film of something oily- butter, mayonnaise, aioli, olive oil, etc.