For jar candles you can take some aluminum foil and wrap it around the outside of the candle. Make sure there is some of the foil that covers the top, but not enough to completely clover the opening (the flame needs to breathe).
The foil reflecting heat will melt more wax and prevent tunneling. Since it is the wax that contains the fragrance, it also makes the candle scent a bit stronger.
The only time I don’t use foil on my candles is when guests are coming over.