Basically to help induce brumation, you want to first make sure your dragons are healthy and a good weight for their size. A vet check and fecal never hurts at this point either.
Next, reduce temperatures and length of day gradually over a couple weeks till daylight ambient temps are around 80-85F (not basking temp, just the ambient temperatures..basking temp shouldn't drop too much, no less than 95F, so your dragon can still bask) and length of daylight is about 8-10 hours. Should take about two weeks to reduce temperatures. Stop feeding your dragon completely by the beginning of the second week, this way all food wil be digested by the time brumation sets in. Offering water daily (via eye dropper, misting or a bath). Let nighttime temps drop to room temperature (try not to let it drop below 60F, but a range between 60-70F is fine for night temps)
By the end of the second week your dragon should be spending most of his or her time sleeping, or digging to set up a place to brumate. Make sure you have hides etc for them to use.
If, after doing all this the dragon's still continue to bask and show no inclination to want to brumate...you will have to bring the temperatures back up and offer food. You can still keep the daylight hours reduced...but you don't want them running about active with no food for long otherwise they will drop in weight. Better to bring the heat back up and allow them to eat.
It isn't necessary to brumate your dragons to enable breeding, and some dragons will brumate on their own, without any change in temperature or daylight.
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PHLdyPayne