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There can be many causes for warp and twist. The secret is to consider them all, so each can be minimized.
From a layout perspective, your stackup should be symetric from the center of the stack out - i.e. one half of the stack should be a mirror of the other. The number of planes should be even and the number of signal layers should be even. The amount of copper should be symetrically distributed on opposing layers. Component locations should take into account thermal inertia when wave soldering. Component locations should also take into account component weight.
From a fabricator's perspective, the stackup should be symetric. The fabric weave bias should alternate and be evenly distributed. The copper should be balanced on opposing layers. Care should be taken not to set press temperature too high. The press temperature up-ramp should be gradual enough not to introduce built-in stresses. The cooldown ramp should be gradual so built-in stress is not introduced.
Component wave soldering equipment should be carefully adjusted to prevent introduction of bow and twist as a result of uneven or rapid heating, or uneven or rapid cooldown. Heavy components should be supported when the board is transported through soldering equipment to prevent sag that can introduce mechanical stress that will result in warp held in place when the solder cools.
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