I use a small ultrasonic cleaner, like the ones used for jewelry and dental bridges, etc, and use a dash of baking soda in hot water from the faucet. I run it twice (3- minute cycles), rinse and replace with plain hot water, and run twice again. Then let them air dry over days of time. I never dry burn because I believe, take on faith, that the ultrasonic cleaner dissolves and shakes off the crusted residue. I check ohms with a meter and if I get a reading and it's somewhere near specs of new heads I'll feel confident and expect clean results. Mostly, that's what I get. If not, it's toss and replace with new.
Most of my juices are coffee and sweet bakery concoctions, dark and tend to gunkiness, 50/50 propylene/vegetable. I only vape one full tankful before doing this cleaning. This frequency, I believe, is key to lack of build-up. It seems to restore the coil to like-newness many times before the effect tapers off. Maybe 6 cycles so far, and more.