Corn tortillas: Make sure they're fresh,fresh,fresh. Ideally, they'd be made that day, but that isn't realistic for most people; just don't buy them days ahead of time. Have them at room temperature before starting to work with them. Have the sauce hot, then dip the tortillas one at a time into the hot sauce, allowing a moment for the the tortilla to soften. If they're very fresh, this may soften it enough that it doesn't crack. If not, then fry it briefly in oil after dipping in the sauce. If you fry it too long or at too high a temperature, it'll become crispy and rolling will be out of the question. Fill and roll immediately. Don't try to roll too tightly; the sides should overlap only slightly. It's more like "fill and fold". If the tortillas are fresh enough, steaming shouldn't be necessary, but if they're still cracking after following the procedure above, it's worth trying.