Depending on the time of year, portions of the salt flat will be covered with a small layer of water, giving you the desired mirrored effect when you take your photos of the horizon. As far as packing, bring clothes you don’t mind getting messy. Any water you get on you will be saturated in salt, which will be seen on your clothing when the water evaporates. Also, the drier parts of the salar will be softer on your feet, as the sand is more powdery. However, walking on a portion of the salar covered in water will be much more painful as the salt you are walking on is in crystalline form, and will feel like walking on jagged rocks.