Remove the wax from a candle jar to reuse the jar
Put it in the freezer for about 10 minutes, then it should pop out. You may have to melt it out though. I did that for my DIY wax melts :)
First, use a knife to pry the candle and any chunks of wax out of the jar. Scrape away excess wax, and remove the wick collar (the metal end of the wick). When the jar is relatively clean, put it in the microwave for about 20 seconds, then wipe away any melted wax with a paper towel.
If the candle was in a glass jar, you can always boil water and then pour the boiling water into the jar. It should melt the wax residue on the jar and you can wipe it right out.
I've done this a few times and I find the best method is to stick the jars in a freezer. After they've frozen, remove the jars and gently use a butter knife to create cracks in the wax. Once you've made a few of them, the wax should pop right out. If the wick bases won't budge, allow the jar to defrost and then fill it with hot water. After a few minutes, the adhesive on the bottom of the wicks should loosen up enough for you to pry them off the bottom of the jar.
Dawn dishwashing liquid & super hot water. The wax becomes loose & you wipe out the jar with a paper towel. Takes about 3 minutes.
After you get most of the wax/residue out, I stick mine in the dishwasher with no other dishes to get them sparkly clean.