They have little candle toppers. that hold the heat in. You can get them at bed and bath or linens and things. They'll get it all burnt.
I usually take my left over candle peices and put them into an empty candle holder. When I get enough to make a candle, I buy a wick and mold the wax to make another candle (with the wick in the center) and voila! I've got another candle.
measure the candle and mark off the half way point; i.e. if the candle is 8 inches long, make a mark at 4 inches. When the candle burns down to the mark, light the 2nd candle. When the 2nd candle burns out it will be 1 and a half hours.