When you barbecue with charcoal, you don't have to start with a fresh batch every time you fire up the grill. All charcoal, whether it's hardwood or briquettes, has a lifespan that may be longer than your grilling session, and you can take advantage of that to reuse it next time. While that lifespan is longer in briquettes -- check your bag for specifics -- you can also relight any charcoal that hasn't burned for as long as possible.
Close up your grill when you're finished cooking. Instead of extinguishing your charcoal with sand or water, closing the grill and its vents allows the charcoal to cool naturally, and makes it easier to relight next time.
Empty the grill after a minimum of 48 hours -- it may take this long for the charcoal to completely cool. Your charcoal burns from the outside in, so while the exterior of a piece of charcoal is ashen and crumbling, the inside could be usable.
Brush your charcoal with a steel-bristle brush to remove the ash from the outside. Alternatively, place it on top of a metal grill or grate, then shake the grill from side to side so that the charcoal rolls back and forth, knocking off the ashen exterior. Place the remaining charcoal into a non-combustible bag or bin.
Refill your grill with new charcoal, leaving room to add the old charcoal you were able to recover. Place the old charcoal on top of the new charcoal.
Lightly soak your charcoal pile with lighter fluid or use a chimney starter, and light it with a match.