First and Foremost: NEVER leave a burning candle unattended, I know we hear that all the time but it really is important!
Always trim the wick to between 1/8 and 1/4 of an inch before lighting. If I've been burning a candle for a couple of hours and see that the flame has gotten big I'll extinguish it, trim it, and relight it. Keeping the wicks trimmed will cut down on the amount of black smoke or soot which is caused by a large flickering flame, and did you know that the lower heat of a smaller flame actually allows more scent to be released?...it's true!
Place your candle in an area that doesn't get a lot of air movement, away from air vents etc., a still flame will allow the candle to burn more evenly.
Try to avoid short usage times, especially true for jar candles, extinguishing your candle before a pool of melted wax has formed all across the top will cause tunneling which means a hollow center and a lot of wasted wax!
When you're all done burning a votive candle, after it has completely cooled, place it in the freezer for about a half an hour, the wax will be easier to chip out. But please be sure to use something that is not sharp or something that will damage you or your votive holder! Then, when most of the wax is out you can run the holder under hot tap water and dry with a paper towel to get it looking just like new.
In the event that a particular taper candle doesn't fit perfectly snug in a certain candleholder, I find that a drip or two of wax in the bottom of the candleholder works well to stabilize the candle. Once the candle is lit, turn it so as to allow wax to drip into the bottom portion of the candleholder then set the candle down in until the wax sets, that should eliminate any wobble. This works in a similar way as the "Stick-Um Candle Adhesive" that has been around since I can remember which my mother used in her crystal candlesticks to ensure the taper candles stood straight and tall.
I love candles and want to share how to get the most enjoyment out of them so I hope you'll find these tips helpful!