Most people just zap a bit of honey in the microwave or heat it in a pan of water on the stove. These methods work well, but you risk overheating and thus burning, darkening and damaging the honey as well as destroying nutrients and enzymes. Beekeepers often use special water-jacketed tanks or heating bands. These are expensive, often unwieldy for small batches and also risk destroying the quality of the honey if not attended closely.
Ideally, to preserve color and quality, you want to hold the honey at about 105 to 115 degrees for several hours, depending on the size of your jars.
My brother and his wife usually get a case of honey from me every year to give to their friends as Christmas gifts. But by Christmas, the raw honey, pulled in the spring, has begun to crystalize in the glass jars. Ingenious man that he is, my brother substitutes the light bulb in his oven with a regular 100-watt bulb and leaves the jars of honey in the oven over night! (Just remember to replace the bulb again before you use the oven to prevent the bulb from breaking! He also suggests checking your oven first by sticking a thermometer in overnight to see how warm your oven gets.)
If I have only a few jars, I use my chicken egg incubator. I can set the incubator at 110 degrees, leave the lids on, and walk away! I can even reliquify comb honey this way because the wax does not melt until about 140 degrees. In the photo, both jars contain cut comb and honey. The one on the right has been reliquified overnight in my incubator.