To avoid a high flame or smoking, ensure to trim the wick down to 5mm above the wax level before each use.
Each time you light your candle, make sure to burn it until the melted wax reaches the edges of the glass, this will encourage even burning, avoid tunnelling and allow you to get more hours out of your candle.
Do not keep your candle in very hot or cold environments. Environments that are too warm will cause the wax to melt too quickly, meaning you will get less burn time and the wick will need trimming more often. Environments that are too cold will cause the wax to become very hard, meaning the wax will not melt properly and your wick will become buried in the candle.
To keep your diffuser smelling as fresh as it did the day you popped those reeds in, simply flip your reed every couple of days.