How to keep square taper cranks tight?
You are undertightening the crank bolts on initial installation. 44Nm or 33 ft-lb work well as generic values for square taper cranks. A basic large beam-type torque wrench works well for crank bolts. If you aren't able to get one, it's safe for basic square taper cranks to reef like mad with something like a 12&quot; ratchet or breaker bar. You'll be exceeding that torque value, but bad things won't happen. Fancier cranks with no extra material to give could eventually crack by doing so, but probably won't.
Always grease the threads and the bolt shoulder. Loctite is not optimal in this application, but it's also sufficient as an assembly lubricant to keep damage from occurring as long as there's appropriate torque.
Once the crank has come loose on the spindle, there's no fixing it. The press interface will never be right again. Throw them away. The spindle is hardened steel and is unaffected in almost all cases. If you're wrenching for others who have this problem, you must not re-install the crank. Doing so is irresponsible because the next time it comes loose, it might happen suddenly and cause a loss of balance and potentially a crash.
Square taper crank interfaces that are correctly installed don't have problems with seizure.
Some perceived loosening of the bolt is normal because as cranks are ridden, they can move around on the spindle enough to decrease thread preload. This is part of the reason why crank bolts are such high-torque fasteners; they must have some extra preload to give to avoid problems from this effect. Jobst Brandt wrote extensively about this on rec.bicycles.tech, material you can find archived by searching around. There are some dynamics happening that aren't readily apparent. Part of the conversation involves whether to grease the taper or not, which I won't get into here because it's outside the scope of the question, and in the end it's fine either way with some qualifiers.