A calm, steady flame:  This means that the candle's burning process is in balance.  The wick is efficiently pulling up the right amount of wax, which is then being "consumed" by the flame to produce water vapor and carbon dioxide.

 

No flickering or wisps of smoke:  Constant flickering and smoking occur when the teardrop shape of the flame is disturbed. This allows small amounts of unburned carbon particles (soot) to escape from the flame as a visible wisp of smoke.  If too much or too little air reaches the candle flame, it will disturb the flame's teardrop shape, causing it to soot.  To avoid this, always burn your candles in a well-ventilated room, away from drafts, vents or strong currents.

 

No flare ups:  A wick that is too long can cause a candle flame to grow too long and flare.  That's why it is so important to trim the wick to 1/4 inch before each use.  Also, always keep the wax pool free of wick trimmings or other debris that could catch fire from the flame. 

 

If you like to group your candles, make sure they are at least 3 inches apart from each other when burning.  Candles placed too closely together can create their own draft and cause the candles to flare.