This usually happens because of remaining solvent residues warming up and redissolving the crystals.
A trick to work around that is to freeze precipitate for a day or so (if you already have the melted crystals, then redissolve them in small amounts of warm naphtha and back into the freezer), and after crystals have precipitated, take it out of the freezer, pour the naphtha off (through coffee filter just in case any crystals are not stuck to the container), close the container and put it upside down in the freezer for some more hours. This way, any remaining naphtha trapped inside the crystals will drop off to the lid of the container on the bottom.
So after those few hours, take out out of the freezer, opening the lid still upside down so the naphtha drops off (remember to save the naphtha from freeze precip to reuse.... ), and then you can scrape the drier crystals, put them on top of a coffee filter or on a plate/petri dish to finish air drying, and voilá
The container used for precipitation is ideally something with a lid that closes well, like a mason jar, and that the edges and walls are straight so you can scrape it off more easily. Scrape as much as you can of the dmt, and whatever is left inside, leave it and reuse the container for the next precipitation, or if its the last precipitation of the extraction, you can use some alcohol (or even naphtha, if its clean evapping naphtha) to pick the last bits up and evaporate in a dish or easy to scrape flat-bottom wide container.
