Thou shalt not poke your meat
You've heard the trick of poking meat and comparing it to the texture of your hand to tell if it's medium, or rare, or burnt, or whatever. The trick works, but only for professionals -- and remember that professionals at this level can tell the doneness of a steak in a pan by the sound it makes. You're not a pro, and you don't want to poison your friends. Buy a damn meat thermometer.
Thou shalt leave the lid alone
Opening the lid of a gas grill reduces the temperature and slows cooking. Opening the lid of a charcoal grill adds oxygen to the fire and burns your meat. Patience, grasshopper. Use a timer, then open the lid to flip, then close it again until it's close to time for the cooking to be done. You don't keep opening your oven and dicking around with a cake or casserole. Resist the urge with the grill.
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