Store paper in a humid environment
You can make sure an equal pressure is applied to the individual papers or blocks while being stored by laying a heavy and flat object like a book or a similar item on top of them. Even if the humidity would otherwise warp the paper, there is no room for that now.
You can try storing your currently warped paper blocks like this for a while, but I doubt they will become completely flush again (they would need moisture so the fibres can expand, and then shrink while the pressure is applied, but the moisture would need a way to evaporate relatively quick or the paper might end up mouldy).
See also the first half of this answer for more information on storing paper.
The already wrinkled or waved paper can be straightened by moistening it and sticking it to a wooden board using water activated tape (see also the second part of this answer). After drying, you can either cut the paper out of the tape borders before using it, or write on it first, so the ink can dry completely without it warping the paper either.