I don't know exactly how "they" bring the temp up slowly, but I have read some posts/articles on DIY Cryo and the method seems to follow something like this:
Freeze as slow as possible too, then:
leave in liquid for as long as it stays cold...
xfer to freezer section of your fridge for another couple days...
(maybe use intermediate step of packing in ice in a cooler, but freezer might be just as cold - test with therometer)
After couple to few days in freezer, xfer to fridge for another day
and lastly, place in cooler by itself until it reaches room temp.
OR - any variation of above that works for you.
Please let us know how it turns out and the method used. You may have a budding new business on your hands.