Each time you light your candle is an investment. To avoid tunneling, make sure that you will be able to turn your candle until the melt pool (liquid area) reaches the edge of the vessel. For 9 oz candles, this will be approximately 1.5 hours.
Before lighting your candle, make sure the wooden wick is trimmed to about 1/8 inch above the wax. This is different from the 1/4 in recommended for cotton wicks. Scissors work great for this, or you can invest in a wick trimmer.
After 4 hours of burning, give your candle a break; otherwise, the vessel may overheat and crack.
Once you've burned out your candle, take the following steps to stay eco-friendly. Put your vessel in the freezer to solidify the remainder of the wax. Once it's frozen, use a spoon or butter knife to pop the wax out - it should stay in nice chunks. Put these leftover wax pieces on your wax melter! Add dish soap to your vessel and wash with warm water to clean out the wax residue. Use a butter knife to pop the wick holder off of the vessel (keep these and take them to a metal recycling location!) Your vessel is now ready to be used for any other purpose or to be recycled.